Because the signal attenuation is great in sea channel so as to seriously impact on the performance of communication system, an excellent error correcting code is necessary to reduce the error rate of data transmission in underwater optical communication. A system model of underwater optical communication is established by combining low density parity-check code with pulse position modulation. A simplified soft demodulation based on the general PPM soft demodulation is used to reduce the complexity of the algorithm, which is convenient for hardware implementation. What's more, the simplified soft demodulation method is very suitable for the extraction of soft information in sea channel because it does not need the detailed features of channel. The error performance between the general method and simplified method is analyzed by using Matlab software. According to the simulation results, the system code gain by using simplified method is decreased because of the simplified extraction of soft information, but it is still higher than code gain of the uncoded system. When PPM order is lower, the performance by using simplified method is even superior to that by Solomon Reed code. On the whole, the scheme of the simplified soft demodulation based on the combination of LDPC and PPM is very suitable for underwater optical communication in huge ranges of applications.