The invention relates to a mechanical damper for nuclear power. The mechanical damper mainly comprises a box body in which a damping system is arranged, wherein one end of the box body is connected with a fixing device, and the other end of the box body is connected with a stroke housing; a stroke rack which can control the stroke of the damper and a synchronizing gear meshed with the stroke rack are arranged in the damping box body; one end of the stroke rack can extend into an inner cavity of the fixing device under a load, the other end of the stroke rack is connected with a stroke housing II, and a speed increasing gear I is arranged in a manner of being coaxial with the synchronizing gear; the speed increasing effect is achieved through the meshing of the speed increasing gear I and a speed increasing gear II; and a damping actuating mechanism I and a damping actuating mechanism II for realizing the effect of retarding an external load are separately arranged on the right side of the stroke rack and the synchronizing gear in the cavity of the box body, and the left side of the speed increasing gear I and the speed increasing gear II. The mechanical damper disclosed by the invention is compact in structure, reasonable in stress, smooth in properties and good in shock absorption effect, and can form soft rigidity support on an instantaneous high-amplitude vibration, so that the safety and the reliability of nuclear power equipment are effectively improved.