The utility model discloses an underwater manipulator dilatable joint for an underwater robot with intense work, comprising a dilatable arm, a square guide sleeve and a dilatable hydrocylinder. The dilatable arm is a hollow cuboid podetium with an even wall and its section is a rectangle. The podetium of one end of the dilatable arm is connected with the front segment joint to drive the front segment joint to conduct concertina movement, the podetium of the other end of the dilatable arm right angled slots are arranged along the margin of the arris at the top corners of the four arris to hold the back angle bearing. The square guide sleeve, consisting of a front baffle, a back baffle, an upper baffle and a lower baffle, is sheathed at the outer surface of the dilatable arm. The dilatable hydrocylinder is disposed inside the chamber of the dilatable arm and the center of the dilatable hydrocylinder shares one vertical plane with the center of the dilatable arm. The back support shaft and the front support shaft are used to connect the dilatable arm and to support the dilatable hydrocylinder. With great weight carrying capacity, long dilation distance, light weight and simple structure, the underwater manipulator dilatable joint can be applied to an underwater robot with intense work.