A MARX cascade circuit laser illumination source for underwater laser range-gated imaging was developed based on the avalanche cascade principle. Compared with the ordinary laser drivers, this MARX cascade circuit had the advantages of narrow output pulse, high peak voltage, short rising edge, high repetition frequency, low cost, small volume and light weight. The pulsed laser illumination source developed by the cascade circuit maintained the miniaturization of illumination source system volume on the basis of realizing the pulse width performance with nanosecond (ns) magnitude. The experiment of range-gated imaging of underwater was carried out by the pulsed illumination source combined with laser shaping and homogenizing optical technology. With a pulse width of 10 ns, the range-gated imaging in the range of 7 times attenuation length under the water was achieved. The improvement of circuits and optics is of great significance to the reduction of volume and cost, and the application of laser range-gated imaging system.