The invention relates to a nuclear power condensate pump mechanical sealing device which comprises a movable ring assembly, a fixed ring assembly, a shaft sleeve and a gland. The movable ring assembly is formed by a movable ring (1), a movable ring base (2), a movable ring base transmission screw (3) and a movable ring sealing ring (23), and a flange (10) is arranged on the movable ring to prevent the transmission screw (3) from escaping out; the fixed ring assembly is formed by a fixed ring (5), a push ring (6), a compensation spring (7) and a spring box (8); a ring sleeve (81) is arranged on the spring box (8) to be matched with a convex groove in the outer ring face of the fixed ring (5) so as to prevent rotation of the fixed ring (5); and a fixed ring base ring sleeve (90) is arranged on the gland to be matched with the inner ring face of the fixed ring (5), and a fixed ring slippage assisting sealing ring (59) is arranged between the fixed ring (5) and the fixed ring base ring sleeve (90). The nuclear power condensate pump mechanical sealing device is good in constitutive property, simple in structure, reliable in transmission, good in fixed ring and movable ring fit floating stability, stable in sealing running, small in leakage amount, safe in use and good in reliability.