A full range-gated imaging system is designed based on the underwater high-repetition-rate range-gated imaging system. In the spatial domain, the whole detecting range can be divided into multiple improved gating slices which are superposed mutually, and the laser pulses during the integral time of one image frame are distributed in these improved gating slices according to certain rules. In the time domain, the echo isotime movement of different laser pulses after the wide gate is achieved according to certain rules, the target information of all distances can be integrated into one frame, and the full range-gated images can be obtained in real-time. The imaging quality of the continuous-wave imaging system, the traditional range-gated imaging system and the full range-gated imaging system is compared. The experimental results show that the full range-gated imaging system has advantages of large detection range possessed by the continuous-wave imaging system and long detection distance possessed by the range-gated imaging system. The detecting distance exceeds 4.5H (H is the decay length) and the frame rate of the output video is 20 Hz.