The invention provides an improved hydraulic fastening device. The improved hydraulic fastening device comprises a cylinder body, the cylinder body is fastened on a fixed base through screws, a piston cavity is formed in the cylinder body, an oil return pipeline is perpendicularly arranged in the cylinder body, one end of the oil return pipeline is parallel to the upper surface of the cylinder body, and the other end of the oil return pipeline is parallel to the piston cavity; a spring and a piston are arranged in the piston cavity, the spring is adjacent to the protruding edge of the piston, supporting rings and a piston seal are both mounted on the piston, a flange cover is fastened on the cylinder body through screws, and an O-shaped sealing ring is mounted at the position where the flange cover and the cylinder body are sealed. The improved hydraulic fastening device has the advantages of being compact in structure, convenient to mount, easy to maintain, convenient and easy to open, excellent in property and capable of satisfying the requirement for production utilization.