文中讨论了激光在海水中的传播特性，指出激光水下目标探测系统所接收到的目标反射回波是一种叠加在后向散射上的微弱光信号，它们的能量是随深度呈指数规律衰减的，且动态范围很大（可达１０￣５）。为处理这种大动态范围的弱光信号，文中研究了ＰＭＴ的增益变化特性，设计出了一种实用的ＰＭＴ变增益控制系统，该系统的控制范围可达１０￣４，并已在一种激光水下目标探测实验系统中得到应用。 In this paper, the propagation characteristics of laser in seawater are discussed. It is pointed out that the target reflection echo received by laser underwater target detection system is a kind of weak optical signal superimposed on backscattering. Their energy decay exponentially with depth Of the dynamic range (up to 10 ~ 5). In order to deal with the weak light signal with large dynamic range, this paper studies the gain variation characteristics of PMT, and designs a practical PMT gain control system. The control range of this system is up to 10 ~ 4, Laser underwater target detection system has been applied.