The invention provides a square pipe die of a pipe extruding machine and relates to the field of pipes. The square pipe die comprises six round cakes, fixing parts, a circular ring, a nut and a screw,wherein the first round cake is provided with a hollow ring; the third round cake is provided with a plurality of fixing holes and the first fixing part; the first fixing part comprises a fixing rodand a fixing head; the fourth round cake is provided with a plurality of second fixing parts; the fifth round cake is provided with a product outlet, a first module and a second module; and the sixthround cake is provided with a plurality of third fixing parts, a plurality of separation parts, hexagon parts and a raw material inlet. Compared with the problems that square pipes produced through existing silicon carbide pipe extruding machines is poor in square pipe wall thickness uniformity and the shapes of the square pipes are liable to be out of circularity, the square pipe die, provided bythe invention, of the pipe extruding machine is mainly designed for machining of silicon carbide square pipes in a targeted mode, and by being connected with the extruding machine, the effects that produced silicon carbide square pipes are uniform in wall thickness, and the shapes of the square pipes are not liable to be out of circularity can be achieved.