The utility model discloses a pressure relief valve of a transformer protective cover, including annular joint spare, pillar and lower extreme open -ended spherical shell body, annular joint spare isfixed spherical shell body's open end, just the annular joint spare the external diameter with the external diameter of spherical shell body's open end equals, the top of pillar is fixed spherical shell body's inner chamber squash top, the valve draw -in groove with pressure relief valve's action valve looks adaptation is seted up to the lower extreme of pillar. The utility model discloses a pressure relief valve of a transformer protective cover can be effectively waterproof dustproof, avoids filthy accumulation, does not influence the normal release function of relief valve simultaneously.