The invention belongs to the technical field of servo mechanisms, and particularly discloses a piston type self pressure complement hydraulic accumulator for a servo mechanism. The piston type self pressure complement hydraulic accumulator for the servo mechanism comprises a shell body. A small piston is arranged on the upper portion of the shell body. One end of a membrane type end socket is embedded in the small piston and the other end of the membrane type end socket is located outside the shell body and connected with one end portion of a gas outlet pipe of a high-pressure gas bottle. A large piston is arranged inside the shell boy, a gas cavity is formed in the upper end of the large piston, and a hydraulic oil cavity is formed in the lower end of the large piston. According to the piston type self pressure complement hydraulic accumulator for the servo mechanism, self pressure complement can be conducted without power supply and control which are conducted independently and therefore the structure of the servo mechanism is greatly simplified.