An object of the utility model is to provide a single plunger type water hydraulic drive pump under water mainly comprises motor, rotation - linear motion shifter, single piston rod -type water pneumatic cylinder, check valve and three -way valve etc.. The motor enlarges the motor torque through reduction gear, rotation - linear motion shifter to turn into pneumatic cylinder piston axial linear motion with the rotary motion of motor, lead screw, the pipeline of forming through check valve, three -way valve converts extraneous water medium suction hydraulic pressure jar to certain pressure water medium supply hydraulic actuator, realizes the hydraulic pump function. The utility model discloses can regard as the rill water gaging hydraulic drive device of underwater operation equipment, small, light in weight, system are simplified, the device has compact structure, simple to operate, through the motor regulation control, realize high accuracy, no pulsatory low discharge hydraulic pressure servo control flow output.