The invention relates to a leak-free rotary fluid damper, which comprises a disc (1), a lower disc flange (10), a hydraulic cylinder (17) and a barrel (19). The lower disc flange (10) is opposite to the disc (1), the disc (1) is connected with the lower disc flange (10) in a leak-free manner by the hydraulic cylinder (17), the barrel (19) is provided with an opening, and the upper end of the barrel (19) is provided with an upper disc flange (32). The lower disc flange (10), the disc (1) and the hydraulic cylinder (17) form a leak-free closed space filled with damping fluid (25), part of the hydraulic cylinder (17) is located in the barrel (19) with the upper disc flange, and second permanent magnets (16) are arranged in the barrel (19) with the upper disc flange. When blades in the damper are driven to rotate by magnetic force, the damping fluid flows through small through holes on the blades to dissipate energy, and accordingly vibration control effect is achieved.