In order to grasp the unconstrained modal characteristics of the flexible airship envelop, the natural vibration performance was investigated thoroughly for a 25 m long stratospheric demonstration airship. The flexible airship envelop features is a freefloating inflatable membrane structure. Based on the mechanical nature of pneumaticstressed membrane structures, the deformed equilibrium shape and stress were computed from the initial geometry configuration, subjected the inflated air pressure through nonlinear structural analysis. The block Lanczos method was employed for modal analysis. The main influencing factors were evaluated thoroughly for modal behavior, including shell element and membrane element, pressure, geometry size, weld seam, nonuniform distributed mass and the added atmosphere mass. The numerical models of membrane and shell element have close natural frequency and identical modal shape. The airadded mass, pressure and geometry size are the key factors to govern the modal performance. This paper can provide reference for structural engineering design of stratospheric airship.