Abstract: Based on the disscusing of laser propagation properties in sea,this paper points out that the recieved energy consists of sea-water backscatter and object reflection,and the later is quite weak,and has a large dynamic range of 105.So,it is necessary to have a gain control system of photomultiplier tube (PMT)to compensate the signal variation.This paper introduces a practical photo-multiplier gain control(PMGC)system,the system has beter dynamic range of 104 and has been used in a un-derwater object laser detection system.