Abstract： In order to reveal the mechanical characteristics of nonrigid airship at the unstable configuration stage, ABAQUS’s user material subroutine based on the iterative membrane properties (IMP) method was proposed. A rational membrane element which was used to simulate the mechanical characteristic of the flexible airship envelop at the unstable configuration stage was chosen by comparing the model results of the engineering elastic beam theory, membrane wrinkling theory and the corresponding experiment. The shape and stress of a 9 meter simplified models of the flexible airship envelop under different constraint conditions was discussed by using the explicit contact nonlinear finite element method.The net buoyant density and boundary conditions are the key factors influencing the mechanical characteristic of the flexible airship envelop at the unstable configuration stage. The nonrigid airship envelop shape varies smoothly at the initial steady state and sharply in the middle of the invariable configuration stage.In addition, the constraint parts of nonrigid airship envelops are more likely to cause strength failure in the invariable configuration phase than in the stable phase.The method proposed is valuable in disclosing the mechanical characteristics of nonrigid airship envelop at the invariable configuration stage.