Influences of the radial internal pressure on the elastic stability (buckling characteristics) of the composite case are analyzed, which based on the Flügge shell theory, in addition, the influences of geometric parameters and material properties on buckling characteristics were also discussed. Results show that the improvement of internal pressure can improve the buckling of casing within the limits of strength. Furthermore, the dynamic responses of the composite case under a radical harmonic force are studied by applying the mode superposition method. It is found that under the radial harmonic excitation, the frequency of the maximum radial displacement is different on the excitation force plane and non-excitation forced plane, more specifically, the maximum radial displacement appears on the third order frequency point of the excitation force plane and on the fundamental frequency of the non-excitation forced plane.