The invention provides a gravity center adjusting device of an operation type underwater robot, relating to the field of underwater robot application. The invention aims to provide a reliable and effective gravity center adjusting device for an underwater robot system, so as to balance the overturning torque generated when a mechanical arm is unfolded or adjust the motion posture of the underwater robot during the operation of the underwater robot. The gravity center adjusting device is transmitted through a primary tapered gear by a step motor; the position of a weight block is adjusted by using the transmission principle of a screw nut; a position detection sensor moves together with the weight block, so as to detect the position of the weight block in a sealed cylinder body in real time. According to the gravity center adjusting device, the effective space inside the underwater robot body is fully utilized and can be used for controlling operation balance and motion posture of the underwater robot, so as to realize pitching or transverse rolling posture control of the underwater robot; the gravity center adjusting device has the advantages of small volume, compact structure, reliability and high adjusting precision and is suitable for operation balance and posture control of the operation type underwater robot.