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.