The invention belongs to the technical field of plastic film extruders, and relates to a filter device of an automatic-deslagging plastic film extruder. The filter device comprises a shell, a filter disc, a rotating device, filter sheets, a filter sheet replacement window, a window cover, a feed port and a discharge port, wherein a feed channel positioned between the feed port and one filter sheet is provided with a deslagging channel; the discharge port of the deslagging channel is dead against the filter sheet which is not dead against the feed port; the deslagging channel or the feed channel is provided with a pressure sensor and a one-way valve; one side of the feed port is provided with a deslagging pipe communicated with the opposite surface of the filter sheet which is dead against the outlet of the deslagging channel; and a controller receives a pressure signal transmitted by the pressure sensor and controls the opening and closing of the one-way valve to achieve automatic deslagging. The filter device disclosed by the invention has the advantages of stability in product quality, long service life of the filter sheets, no raw material waste, non-stop automatic deslagging, time saving, high efficiency, low cost and suitability for plastic extrusion equipment of various types.