A solid rocket engine inner insulation layer forming method comprises the following steps: 1, evenly coating the inner wall of a solid rocket engine casing with an adhesive, drying in air, pasting aninsulation layer rubber sleeve onto a corresponding bonding position on the inner wall of the solid rocket engine casing; 2, putting a gas bag fully filled with a gas into the solid rocket engine casing with the insulation layer rubber sleeve, mounting positioning fixtures on both ends of the solid rocket engine casing, and putting the solid rocket engine casing into a vacuum container; 3, vacuumizing the vacuum container for expansion of air in the gas bag under vacuum conditions so as to apply 0.1MPa pressure to the insulation layer rubber sleeve pasted on the inner wall of the solid rocketengine casing to complete solidification of the adhesive so as to complete pasting forming of the insulation layer rubber sleeve and the solid rocket engine casing; and 4, taking the solid rocket engine casing out of the vacuum container, removing the positioning fixtures on the both ends of the solid rocket engine casing, and taking the gas bag out. According to the method, a gas bag pressurizingmedium is changed from a high-pressure compressed gas to an ordinary pressure gas, production energy consumption is reduced, the sealing treatment process on the casing in the adiabatic process is omitted, and production operation difficulty is effectively reduced.