The utility model discloses a pretreatment ash crushing net of an SCR denitration device. At present, after entering an SCR denitration reactor, large-particle dust combusted by a boiler is prone to being deposited on the surface of a catalyst, and local blockage of the catalyst is caused. The utility model discloses a pretreatment ash crushing net of an SCR denitration device. The device comprises a base, the device comprises an ash blocking net (1) and an annular supporting frame (2), the perimeter surface of the ash blocking net is welded on the inner side annular surface of the annular support frame; one group of ash blocking nets adopts a modular structure; the ash blocking net is arranged at the position of an outlet flue (4) of the economizer (3), the other end of the ash blocking net is connected with the reactor, a set of insertion holes (5) are formed in the annular supporting frame, a set of threaded rods (6) are welded to the annular face of a flue opening of the outlet flue, the threaded rods are inserted into the insertion holes of the annular supporting frame, and fastening bolts (7) are screwed into the threaded rods. The utility model is applied to an outlet flue of a boiler economizer or an inlet flue of an SCR denitration device.