The invention relates to a device and method applicable to pleated HEPA filter paper of a square air-conditioner filter, and belongs to the technical field of vehicle engineering. The device mainly comprises a base, a positioning mechanism for clamping and fixing the square air-conditioner filter is arranged on the side of the base, and a pleating mechanism is arranged above the base; the pleating mechanism comprises a top plate and a pressing plate which is in sliding fit with the top plate, the top plate is provided with an empty groove, a plurality of pleating blocks are arranged inside the empty groove, and the upper and lower ends of the pleating blocks protrude out of the top plate; the width of the adjacent pleating blocks is identical; the two sides of any pleating block are fixedly connected with the top plate through springs; the bottom of any pleating block is in a V shape, and the pleating blocks move downwards under the effect of the pressing plate. By designing the brand-new structure, the HEPA filter paper can be profiled and pleated on the basis of existing air-conditioner filters, and it is ensured that the pleated HEPA filter paper can be well attached to the surface of the air-conditioner filter.