As the most frequently used material in stratospheric airship, the mechanical properties of envelope fabrics that are significant for the structural analysis have attracted widespread attention. Biaxial cyclic tensile load with eleven stress ratios were applied to estimate tensile properties of envelope fabrics. Based on the stress-strain curves of eleven stress ratios, polynomial fitting provided a reliable and convenient method to describe the biaxial tensile behavior of envelope fabrics. Besides, the response surfaces of stiffness and Poisson's ratios were calculated by the least-square method and fitted by binary quadratic polynomial. The accuracy of polynomial fitting method was analyzed, and the elastic properties of envelope fabrics provide reference for engineering application.