Wireless optical communication system based on low density parity check (LDPC) codes and multi-pulse position modulation (MPPM) is established, and the decoding algorithm is introduced according to the system model. Bit-symbol mapping method of (m,2) MPPM is studied, and a novel bit-symbol mapping method is presented based on modified genetic algorithm(GA), which is simulated under turbulence channel. The simulation results show that LDPC codes have excellent error correction capabilities with MPPM, and the wireless optical communication based on multiple-beam propagating technique and LDPC codes can operate under a strong atmospheric turbulence. Bit-symbol mapping method optimized by GA can get more coding gain without increasing complication.