In order to reduce the molding defects of composite shell and optimize the winding process,the change of internal porosity of EPDM insulation under lower vulcanization molding pressures was observed by microscope. Influence of vulcanization molding pressure on the body performance and interface bonding performance of EPDM insulation was studied.The results show that, when the vulcanization molding pressure is less than 0.15 MPa, there are more pores in the insulation after vulcanization with lower density,mechanical properties and ablative properties.With the increase of molding pressure,insulation porosity gradually decreases. Insulation performance and interface bonding reach a high stable value to meet the requirements of use.