Atmospheric turbulence can greatly degrade the performance of Free Space Optical (FSO) Communication system. With Gauss channel model and threshold detection, studies about Pulse Position Modulation (PPM) under turbulence have limitation in describing its characteristic. Mathematical calculation expressions about Symbol Error Rate (SER) and Bit Error Rate (BER) of PPM under turbulence channel based on Maximum-likelihood Detection (MLD) are deduced. Monte Carlo methods verify the presented theoretic calculation models. Simulations show that, PPM based on MLD will present better performance, which is also easily influenced by turbulence. The PPM receiver requires average received Signal Noise Ratio (SNR) gain not less than 0.67 dB when the modulation level is increased one level with BER 10-6 in weak turbulence atmosphere environment. For fixed peak power PPM system, synthesized devices capability and communication conditions, proper compromise should be made between the error performance and the power efficiency.