The invention discloses a three-stage underwater deformation robot. The robot comprises a main body section, a front deformation section and a rear deformation section; the front deformation section and the rear deformation section are located at the two ends of the main body section respectively, and can move back and forth relative to the main body section; a slideway is arranged on the main body section, and row hole sliding supports matched with the slideway are arranged on the front deformation section and the rear deformation section respectively. According o the robot, by means of a propeller-driven deformation mode, the posture can be conveniently and rapidly changed underwater without additionally arranging a deformation mechanism, the control complexity is greatly reduced, and unnecessary hardware and software resource waste is reduced. In addition, underwater double-arrangement infrared short-distance measurement is adopted, judgment of the extension or contraction directionof the deformation length of the underwater robot can be achieved without additionally arranging the complex control mechanism, and the accurate deformation length is obtained; the robot has the advantages of being low in cost, high in stability, low in power consumption and the like.