Abstract: In order to improve the problem that the process window for the size of the silicone rubber mandrel was too narrow, which led to the high sensitivity of the forming quality of the composite hat-stiffened structure, a novel mandrel pressurization method combining silicon rubber mandrel and vacuum bag airbag was proposed, and the pressure distributions in the co-curing process and the forming accuracy, microstructure and mechanical properties of the composite hat-stiffened structures formed under the combined mandrel with different adjustable apertures were studied. The results show that the internal pressure fluctuates obviously and unevenly without the opening of adjustable aperture. With the increase of aperture proportion X_S, the internal pressure is uniform and stable at the required pressure of 0.6 MPa within X_S=0.40-0.53. At the same time, the average differences between the component thickness and the cavity height areonly 0.046 mm and 0.40 mm. The microstructure quality of the triangular area is high, and the average pull-off performance and increase rate are 3.42 MPa and 23.02% respectively. The method proposed in this study has a wider process window of adjustable aperture, which has a certain application potential in the forming manufacturing of composite hat-stiffened structures.