Abstract: The form finding analysis of tensile membrane structures by the force density method does not reflect the nonlinear deformations in the length or width directions of the membrane areas represented by membrane links, leading to some differences between the numerical result and the real solution. This paper presents an improved nonlinear force density method taking into account for the effect of 2-dimensional deformations of the membrane surface. The membrane deformations in both length and width are considered during the calculation process. The computation program is also designed using Matlab and its data formats are compatible with Easy, a professional program for the analysis of cable and membrane structures. It is proved that the result obtained by the proposed method is closer to the theoretical solution through the comparison example. Finally this method is employed successfully in the analysis of the super-huge tensile cable-membrane roof of the Expo Axis project for Expo Shanghai 2010.