The characteristics of underwater laser channel are introduced and the receiving model of laser PPM signal is analyzed.Then beginning with information theory,the effects of channel parameters such as pulse time-domain extension,unit pulse power,and the number of time-slots transmitting information per unit symbol interval on the channel capacity of laser PPM have been studied by adopting soft decision demodulation in certain background noise.The simulation indicates that both the channel capacity and information transmission rate decrease with pulse width broadening,increase with power enhancing.And still the two parameters present a trend that they first increase then decrease and finally approach to zero when the transmission time-slot number becomes larger.So there is an optimal time-slot number that makes the channel capacity and information transmission rate reach a maximum respectively and provides theoretical basis for the design of underwater laser communication system.