Taking the experimental apparatus of dual rotor?bearing?casing aero?engine as the reference, the finite element model of casing and dual?rotor is established by using truncated conical shell element method and Timoshenko beam theory, separately, and the finite element model of whole aero?engine experimental apparatus has been got. The influence of casing parameters on whole aero?engine is studied. The research shows that with the increasing casing?rotor mass ratio, the vibration amplitude of casing decreased first and then increased. From the standpoint of optimizing the vibration amplitude of whole aero?engine, there is an optimum mass ratio. When casing?rotor mass ratio is about 0.45, the vibration amplitude is minimum. Different casing material has negligible effects on the critical speed, but it can influence the vibration amplitude of whole aero?engine.The vibration amplitude is minimum when alloy steel casing is used, and the vibration amplitude is maximum whenaluminum alloy casing is used.