The invention relates to a screen-replacement-free gas filtering device which comprises a rectangular shell, two cavities are formed in the shell, air outlet holes are formed in the front side and the rear side of each cavity, a through hole is formed in the middle of the shell, a sliding block is arranged in the through hole, each cavity is provided with two arc-shaped plates, upper air holes are formed in the left side face and the right side face of each sliding block, and lower air holes are formed in the lower end face of each through hole. An air inlet is formed in the lower end face of the shell, the sliding block slides to any end of the through hole, the two arc-shaped plates at the corresponding end can block the air outlet, the upper air hole is communicated with the lower air hole, the air inlet is closed, the air inlet at the opposite end is opened, back-flushing cavities are formed in the upper ends of the two cavities, filter screens are fixed to communicating openings of the back-flushing cavities and the cavities, and piston plates are arranged in the back-flushing cavities; the sliding block is pulled to move through deformation of the arc-shaped plate, the filtering area is increased and the filtering time is prolonged through deformation of the arc-shaped plate while non-stop screen replacement is achieved, meanwhile, the blocked filter screen is subjected to back flushing, and the screen replacement and back flushing efficiency is greatly improved.