The invention discloses a pressure self-balance type electromechanical servo action device under the high-pressure environment. The pressure self-balance type electromechanical servo action device comprises a servo motor, a planet pin roller screw pair, a piston, a speed reducer, a sensor, a shell and other matched devices. The shell is internally fully filled with oil liquid, and the piston can freely move in a piston cylinder. An oil passing hole is reserved in a main shell, and it is guaranteed that oil liquid in the main shell and a front shell flows smoothly. The rotation movement of a screw is converted into the linear movement through a nut, and an output rod is pushed to output the linear movement. A linear displacement sensor is arranged on the main shell, and a magnetic ring is arranged on a guiding block and does the linear reciprocating movement along with a deep groove ball bearing. A high-low lip seal is adopted as a dynamic seal at the position of a copper sleeve, static seals are adopted at other shell connecting parts, and inner and outer liquid mixing is prevented. The pressure self-balance type electromechanical servo action device solves the problems that an existing electromechanical action device is weak in voltage endurance capability, low in efficiency and the like, the power to weight ratio, the voltage endurance capability and the load capability of the electromechanical action device are improved, and a technology guarantee is provided for promoting the development of submarines and underwater devices under the high-pressure environment.