The invention provides an obstacle detection system and method for an underwater robot, and the system is composed of a robot body, a ground control box and an umbilical cable. The robot body is provided with a multi-beam image sonar, a high-definition camera, an adjustable illumination lamp, a motion controller, a pressure sensor, a temperature sensor, an attitude compass and a UDSL converter. The ground control box comprises an image processing unit and a motion control instruction generator. The system introduces an underwater sonar system, a camera system and an image processing device forintelligently recognizing obstacles in front of the underwater robot, and can process the sonar image and the camera image in real time, detect and calculate the size and distance of underwater obstacles, facilitates the improvement of the obstacle avoidance ability of the underwater robot and reduces the damage to the robot.