The Intensity Modulation/Direct Detection (IM/DD) system is simple and easy to implement, which is most widely used in Free Space Optical (FSO) communication. Its typical modulation schemes include On-off Keying (OOK), L-level Pulse Position Modulation (LPPM), Differential Pulse Position Modulation (DPPM) and Multiple Pulse Position Modulation (MPPM), Digital Pulse Interval Modulation (DPIM), etc. OOK is the simplest modulation scheme to implement, but the power utilization is too low and the anti-interference ability is poor. LPPM has superior power utilization at the expense of a large amount of bandwidth. Also the transmission capacity is difficult to meet the requirements of FSO communication, and strict symbol synchronization is required during demodulation, which increases the complexity of the system. DPPM improves the bandwidth utilization relatively and does not require symbol synchronization. But redundant“0”time slots are still generated in the coding process, which makes this modulation scheme not high enough in transmission capacity and bandwidth utilization. MPPM further improves the bandwidth utilization, nevertheless the error performance is poor. And strict symbol synchronization is required which increases the complexity of the system. In order to improve the performance of the traditional PPM schemes, a Novel Differential Pulse Position Modulation (NDPPM) scheme is proposed in this paper, combining with the advantages of DPPM without symbol synchronization and high bandwidth utilization of MPPM.A novel differential pulse position modulation scheme named NDPPM is proposed in this paper. The mapping relationship and symbol structure of NDPPM are studied, the modulation performance is analyzed and compared with other modulation schemes. The average time slot error rate and packet error rate models of FSO communication system under Gamma-Gamma turbulent channel are derived. According to the derived models above, the error performance simulation is carried out. The influence of turbulence intensity, transmission distance and modulation order on the error performance of NDPPM system is analyzed, and the error performance of NDPPM and several other modulation schemes is compared and analyzed.NDPPM does not require symbol synchronization like DPPM, and its transmission capacity is about 4 times that of LPPM and more than 2 times that of DPPM and MPPM for a larger n. The bandwidth requirement of NDPPM is second only to MPPM, which is 1/4 of LPPM and half of DPPM for a larger n, but the transmission capacity, power utilization and error performance are better than that of MPPM. Also compared with MPPM, NDPPM does not require symbol synchronization, which can simplify the complexity of system implementation. Through the simulation of NDPPM system and channel parameter, it can be seen that the increase of turbulence intensity or transmission distance will lead to the decrease of NDPPM system performance, but under different turbulence intensity, the change of transmission distance has different effects on packet error rate performance. Under weak and medium turbulence conditions, the change of transmission distance has a great influence on the performance of packet error rate, while under strong turbulence condition, the influence of the change of transmission distance on packet error rate performance becomes smaller .The reason is that when the turbulence is strong, the turbulence is the main factor affecting the performance of packet error rate, while in weak and medium turbulence, the change of transmission distance is the main factor affecting the performance of packet error rate. The performance of NDPPM system can be improved by increasing the modulation order or Signal-to-Noise Ratio (SNR). If when the packet error rate is 10^-6, every 1 increasing in modulation order, 2 dB in SNR can be saved at least.Through the analysis of NDPPM code pattern, its modulation performance is derived. Compared with the traditional PPM schemes, NDPPM has the highest transmission capacity, and its bandwidth requirement is lower, second only to MPPM. Also NDPPM does not require symbol synchronization, which can simplify the complexity of system implementation. The error performance of NDPPM is significantly better than that of MPPM and OOK. When Cn2=9.0×10-15m-2/3, n=7 and the packet error rate is 10^-6, its performance is superior to MPPM and OOK at 4 dB and 10 dB , respectively. And the performance advantage becomes more apparent as the modulation order increases. The error performance of NDPPM is not as good as that of DPPM, nevertheless, higher modulation order can be adopted to obtain better performance, e.g. when Cn2=4.0×10-14m-2/3 and μ₀=10 dB, the packet error rate of NDPPM with n=7 is one order of magnitude lower than that of DPPM with n=5. The simulation results of NDPPM system show that the increase of turbulence intensity or transmission distance can lead to the decrease of its system performance, while the influence of the change of transmission distance on packet error rate performance is different under different turbulence intensity. The performance of NDPPM system can be improved by increasing the modulation orders or SNR. Considering the complexity of system implementation and performance comprehensively, NDPPM has some advantages and applications in FSO communication. Also the appropriate modulation scheme should be adopted according to the actual demand in the application.