A novel method to determine in-plane shear properties of flexible composite skins with large deformation is proposed in this paper. A geometric model of flexible composite skin is first established, and a formulation is then derived to calculate in-plane shear properties of flexible composite skins with large deformation. After this, a test fixture is prepared to determine in-plane shear properties of 1100dtex polyester fiber/3L rubber composite sheet with large deformation. Experiments show that (1) the test method and fixture are effective to clamp the specimens, attributing to uniform stress distribution in deformation zone and good consistent load-displacement curves. (2) 1100dtex polyester fiber/3L rubber composite sheet displays distinct nonlinear shear characteristics. Shear modulus increases nonlinearly with shear strain, but the growth rate of modulus exhibits an overall monotonic decline as shear strain increases.