The method for analyzing the deformation of flexible skin under the air loads was developed based on the panel method and finite element method. The deformation of flexible skin under air pressures and effects of the local deformation on the aerodynamic characteristics were discussed. Numerical results show that the flexible skin on the upper surface of trailing-edge will bubble under the air loads and the bubble has a powerful effect on the aerodynamic pressure near the surface of local deformation. Then the stiffness requirements for flexible skin of variable trailing-edge were given by using the Jacobs rule, i.e., the maximum displacement of skin is not greater than 0.1% of wing chord. Results show that the in-plane stiffness can be reduced by increasing the ratio of bending stiffness to in-plane stiffness. Although the deformation of flexible skin increases with the in-plane stiffness decreasing, it depends on the bending stiffness. When the bending stiffness exceeds critical value, the deformation of flexible skin only depends on the bending stiffness and has nothing to do with the in-plane stiffness. The conclusions can be used for the structural design of flexible skin.