In this paper, an enhanced optical spatial modulation (EOSM) system with a variable number of activated lasers is proposed to solve the problems of low transmission rate and laser utilization in the traditional optical spatial modulation system. The spatial domain mapping is increased by activating index combinations of one or two separate lasers each time. The characteristics of pulse position modulation (PPM) are specifically utilized to distinguish the various types of mapping. In this work, a detailed discussion of the mapping rules of the spatial domain and the signal domain is given. The theoretical upper bound of the bit error rate (BER) of the EOSM system for the weak turbulence channel is derived by using the joint bound technique. Furthermore, the performance of the proposed system is compared with that of three existing optical spatial modulation systems. The results show that the transmission rate of the EOSM system is greater than those of the spatial pulse position modulation (SPPM) and spatial pulse amplitude modulation (SPAM) systems when the number of lasers and the modulation order are fixed. Considering a transmission rate of 6 bit·s ^-1 and a modulation order of 4, the BER of EOSM system is similar to that of the SPPM system, but it is significantly better than those of the SPAM system and the generalized spatial pulse position modulation (GSPPM) system. When the BER is 10 ^-3, the signal-to-noise ratio of EOSM system is improved by 4.5 dB and 1.2 dB compared with those of the SPAM and GSPPM systems, respectively. The computational complexity of the EOSM system is 17.78% and 2.6% higher than those of the SPAM and GSPPM systems, respectively, and 70.2% lower than that of the SPPM system. Moreover, the EOSM system can effectively improve the utilization of the laser and greatly reduce the construction cost of the system.