The utility model relates to a grinding device and in particular relates to an automatic pressure relief device of a powder wet-process roller mill. The automatic pressure relief device comprises a roller (1), wherein a charging hole (2) is formed in the roller (1); the automatic pressure relief device is characterized in that a through hole (3) is formed in the roller (1), wherein a slide block sheath (5) which is in interference sealing fit with the through hole is arranged in the through hole (3); a slide block (4) which is in clearance fit with the slide block sheath (5) is arranged in the slide block sheath (5); a water vapor inlet (5a) is formed at one end of the slide block sheath (5); and a water vapor outlet (5b) is formed at the other end of the slide block sheath (5). The automatic pressure relief device disclosed by the utility model has the advantages that a pressure relief device is additionally arranged on the base of the original device, and the roller can release the pressure once while rotating by a circle. Compared with the prior art, the automatic pressure relief device disclosed by the utility model has the advantages of being capable of continuously working, time-saving, high in working efficiency and the like.