Abstract: The carbon fiber/bismaleimide resin composite circular tubes were processed by the thermal expansion molding using silicone rubber, based on the establishment of the formula for calculating the dimensions of the silicone rubber mandrel and the process gap. The effects of the thickness of silicone rubber mandrel on the distribution of the temperature and resin pressure variation inside the prepreg stack during processing were investigated, and the manufacturing quality of the composite tubes was analyzed. The results show that the thickness of the silicone rubber mandrel strongly influences the distribution of temperature, and the temperatures are more uniform for the silicone rubber mandrel with 5 mm thickness. Moreover, the resin pressures in the prepreg stack can reach the designed values, but the moment of applied pressure and the pressure level are not identical at different locations. In addition, the manufactured carbon fiber/bismaleimide resin composite circular tubes have good surface quality and uniform thickness distribution without obvious defects inside, demonstrating the feasibility of the designed mold scheme.