To solve the resonance problem of an aviation parallel casing, the finite element method is used to analyze the constrained modal frequencies, considering typical working conditions and the elastic support stiffness of the aircraft frame. This study investigates the influence of casing structural parameters on the constrained modal frequencies. The results show that optimizing the constraint position is the most effective way to significantly change the natural frequencies. 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 reach their minimum and decrease obviously with a reduction in the constraint distance. With an increase of the thin-wall thickness, the 1^st, 3^rd, 5^th and 6^th modal frequencies decrease, while the 2^nd and 4^th modal frequencies increase. The 1^st and 3^rd modal frequencies are almost unaffected by the stiffener parameters. The 2^nd and 4^th modal frequencies increase with the increasing width and height of the stiffener and decreasing stiffener aspect ratio. In addition, the 4^th modal frequency increases with the increasing angle of the stiffener. The 5^th and 6^th modal frequencies increase with an increasing stiffener angle and decreasing stiffener width. Based on these findings, the aviation elastic parallel casing was optimized to ensure the natural frequencies avoid the resonance intervals, thus improving the vibration resistance of the casing.