The utility model discloses a fixed pressure relief device for powder wet drum mills, which comprises a drum (1); and the device is characterized in that the wall of the drum (1) is provided with a through hole (2), and a sheath (6) which is in interference seal fit with the through hole (2) is arranged in the through hole (2); a movable top tube (4) which is in clearance fit with the sheath (6) is arranged in the sheath (6), and the movable top tube (4) is provided with a backing pin (5); the outer side of the sheath (6) is sleeved with a spring (7), one end of the spring (7) is in contact fit with the drum (1), and the other end of the spring is in contact fit with the backing pin (5); one end of the movable top tube (4) in the drum is provided with a water vapor inlet (4a) and connected with a seal baffle (8), and the seal baffle (8) is in seal fit with the inner wall of the drum (1); the other end of the movable top tube (4) is provided with a water vapor outlet (4b) and connected with a collision ball head (9); and a rack of the drum (1) is provided with a fixed ball head (3), and the fixed ball head (3) is in collision fit with the collision ball head (9). Because a pressure relief device is added on the basis of an original device, pressure relief operation can be performed once when the drum turns for a circle, therefore, compared with the prior art, the device has the advantages that the device can run continuously, and is time-saving and high in working efficiency, and the like.