There is provided a multi-turn hydraulic actuator used to open/close a passageway in ground and sea industries including general industrial machines, oil refineries and chemical plants and power generating plants, among others, and more particularly, a multi-turn hydraulic actuator comprising: a hydraulic motor and a decelerator, among others, whereby high torque is efficiently obtained by using low electric power, compared to a conventional electric actuator, and whereby an operation part and a drive part are freely attached to/detached from at a user's necessity.In the multi-turn hydraulic actuator, the hydraulic motor is driven by using hydraulic pressure generated by a hydraulic pressure generator, a rotational force of the hydraulic motor is transmitted to a worm shaft of the decelerator directly connected to the hydraulic motor so that a driving shaft is rotated in a reduced speed, and the rotation of the worm shaft is selectively manually operated by setting a manual mode switch and using a manual handle.