The invention discloses an automatic film replacing device of a particulate matter sampler. The device comprises a blank film barrel, an upper film opening, a sampling film barrel, a sliding groove upper plate, a sliding groove lower plate, a lower film opening, a compression spring, a film jacking support, a film jacking rod, a first guide rail, a direct current motor, a motor base, an automatic cam, an anti-falling sliding plate, a lead screw motor, a second guide rail, a push plate, a filter film clamp, extension springs, a back base plate and a cam plate. A sliding groove upper plate and a sliding groove lower plate are fixed to the back bottom plate, a blank membrane cylinder, an upper membrane opening and a sampling membrane cylinder are sequentially installed at the upper end of the sliding groove upper plate, a plurality of filter membrane clamps are arranged in the blank membrane cylinder, a sliding groove is formed between the sliding groove upper plate and the sliding groove lower plate, and a lead screw motor with a lead screw and a second guide rail are installed in the sliding groove. According to the invention, the push plate is matched with the anti-drop sliding plate, so that the filter membrane clamp can be automatically replaced, and the sampling efficiency and the sampling precision are improved; due to the arrangement of the non-return membrane buckle, the sampled filter membrane clamp is not easy to fall off.