The invention discloses a thin-wall cavity box fast-sealing method and device. The device comprises an electric cabinet box, a filler port, a sealing plate, a sealing lock, double sealing strips, a flexible pipe and the like. According to the method, the flexible pipe is filled with gas or liquid through the filler port, positive pressure is achieved, and the flexible pipe expands and deforms, so that a sealing effect on the sealing plate and the electric cabinet box is achieved and isolation from the outside is achieved. The thin-wall cavity box fast-sealing method and device achieve the fast-sealing effect and can be used for sealing boxes which are differently shaped.