In order to solve the resonance problem of an aviation elastic thin-walled casing, the finite element method was used to analyze the constrained modal under typical working conditions based on the elastic support stiffness of the aircraft frame. In this paper, the influence of structural parameters of aviation elastic thin-walled casing on the natural frequencies of constrained modal was studied. The results show that optimizing the position of constraint is the most effective way to change the natural frequencies greatly. When the natural frequency is at the edge of the resonance intervals, it can be adjusted slightly by optimizing the wall thickness and stiffener parameters. When the two constraint positions are located on the symmetry axis of the casing plane, the natural frequencies are minimum and decrease obviously with the decreasing of the constraint distance. With the increase of the thin-wall thickness, the 1st, 3rd, 5th and 6th modal frequencies decrease and the 2nd and 4th modal frequencies increase. The 1st and 3rd modal frequencies are almost not affected by the stiffener parameters. The 2nd, 4th modal frequencies increase with the increasing width and height of stiffener and decreasing stiffener aspect ratio. In addition, the 4th modal frequency increases with the increasing angle of stiffener. The 5th and 6th modal frequencies increase with increasing stiffener angle and decreasing stiffener width. On this basis, the structural parameters of the aviation elastic thin-walled casing were optimized to make the natural frequencies avoid the resonance intervals, thus improving the vibration resistance of the casing.