<p>To reveal the variation of shear strength of unsaturated soils more intuitively, entirely and precisely, a nonlinear envelope shell model for shear resistance of unsaturated soil was put forward considering the fact that the shear resistance develops nonlinearly with the net normal stress acting on the failure interface and that the suction-attributed friction angle varies nonlinearly with the matric suction. Accordingly, based on the generalized shear strength criterion, van Genuchten’s soil-water characteristic curve model and Vanapalli equation, the calculation scheme for the unsaturated shear strength was presented. The proposed envelope shell is a nonlinear improvement of Fredlund’s planar envelope, and it is able to sketch the dual nonlinearity characteristics of shear strength of unsaturated soil in both dimensions of matric suction and the net normal stress. The applicability and reliability of the suggested shell model for nonlinear shear strength of unsaturated soil were verified by comparing with several cases from published literatures, with relative errors varying in the range of -6.31% ~ 1.14%. By adopting the envelope shell model, the shear strength of unsaturated soil could be better estimated, especially for a broad scope of normal stress or matric suction.</p>