Abstract: The concept of stiffness ratio is defined to study the strengthening of a fuselage with large opening, in order to meet the design requirements of stiffness and deformation. The relationships among the opening angle, the fuselage radius, the skin thickness, the beam area and the stiffness ratio are formulated through a theoretical analysis. The principle and the method to strengthen the fuselage with large opening are successfully applied on the aircraft in the preliminary design stage and can be used to guide the structure design.