The invention relates to a fully sealed pneumatic flat valve. The fully sealed pneumatic flat valve comprises an upper valve body, an inlet pipe, a lower valve body, an air cylinder and an air cylinder support, wherein the inlet pipe communicates with the upper end of the upper valve body, fixing rings are arranged on the lower sides of communication ports of the inlet pipe and the upper valve body, the upper ends of the fixing rings surround the communication ports of the inlet pipe and the upper valve body, the lower end of the upper valve body communicates with the lower valve body, the upper side of the communication port of the upper valve body and the lower valve body is provided with a sliding way, the air cylinder is fixed on the upper valve body through the air cylinder support, astretchable rod of the air cylinder is hinged to one end of a crank handle, the other end of the crank handle is connected with a rotating shaft, the rotating shaft penetrates through the upper valvebody and is connected with one end of a flat plate, the flat plate is located between the fixing ring and the sliding way, the cross section of the other end of the flat plate is larger than the shape enclosed by the sliding way, meanwhile, the cross section of the end is larger than the shape enclosed by the fixing ring, the flat plate is tightly attached to the lower end of the fixing ring andthe upper end of the sliding way, and the flat plate can slide on the sliding way. The economical and practical fully-sealed pneumatic flat valve is provided.