The invention discloses a center of gravity adjustment system and method of an underwater robot. The gravity adjustment system of the underwater robot center comprises a base, a supporting seat, a balancing mechanism, a rotating platform, a load-bearing platform, a center of gravity slightly-adjusting mechanism and a plurality of load-bearing blocks. The load-bearing platform is slidably installedon a first sliding block on a first single-shaft sliding table, a pair of supporting arms is arranged in parallel at the bottom of the load-bearing platform, the supporting arms are slidably arrangedin a sliding groove, and the load-bearing platform is arranged at the telescopic ends of three telescopic mechanisms. The center of gravity slightly-adjusting mechanism is arranged at the top of theload-bearing platform, a second sliding block is arranged on the center of gravity slightly-adjusting mechanism, and a lifting mechanism is arranged on the second sliding block. The load-bearing blocks are slidably arranged at the upper end of the load-bearing platform, the load-bearing blocks are erected at the upper end of the center of gravity slightly-adjusting mechanism, a penetrating hole isformed in the center of each load-bearing block, the inner diameter of each penetrating hole is not less than the outer diameter of the lifting end of the lifting mechanism, and the lifting end of the lifting mechanism selectively extends into the corresponding penetrating hole. According to the center of gravity adjustment system and method of the underwater robot, the center of gravity of the robot can be adjusted to meet the work requirements of the robot, and the technical problem that the center of gravity of the underwater robot is not prone to being adjusted is solved.