Abstract: A novel range-gated gain-control photomultiplier tube (PMT) detector system was designed in order to detect the backscattering signal under water dynamically in real time. The gating of PMT was realized with the method of adding the applied gating and gain-control nano-second pulse, and by changing the electric potential difference between the cathode and the first dynode. The gain control of PMT was complemented by the aid of adding the high voltage pulse to the last dynode of PMT to change the electric potential difference between the last dynode and front dynode. The experiment result shows that, when the gating and gain-control pulses come, the anode outputs the relevant pulse signal whose pulse-wide is 40 ns. It proves that this method can realize the gating and gain-control of PMT.