The invention discloses a single screw extruder for forming a propellant with a high solid content. The single screw extruder comprises a cylinder, a screw, a feeding hopper and a transmission system, wherein the cylinder is formed by a cylinder body steel sleeve and a groove bushing; a rectangular metal casing defecting design is carried out on the cylinder body steel sleeve in the axial direction and the circumferential direction, i.e. special pressure and explosion relief windows are formed; rectangular through grooves parallel to the axial center line of the cylinder body and evenly distributed around the circumference of the cylinder body are formed on the inner side and the outer side of the groove bushing, so that pressure and explosion relief walls are formed; the screw is an isometric columnar mutation type screw with double heads; a core hole through which heating water passes is formed in the center of the screw; and the screw is composed of a feeding segment with a relatively deep spiral groove, a metering segment and a mutating segment with short length. The single screw extruder provided by the invention can be used for safely, effectively and stably forming the propellant with the high solid content in an extrusion manner.