The stress of the soil mainly for unloading instead of regular loading in the process of excavating foundation pits, but conventional loading test methods were taken to determine the soil parameters and applied to the numerical analysis and design calculation for a long time. The stress-strain curve of the soil masses is nonlinear, so there is a great difference between the loading modulus and the unloading modulus of the soil masses, which has great impact on the deformation of soil masses. During the pit excavation, the soil stressstrain relationship is also affected by the stress path. The paper deduced the expression of unloading modulus according to unloading stress-strain curve relationship expressed in form of the hyperbolic, and explored the relationship between the initial unloading modulus and initial deviation stress, then established a nonlinear geotechnical material constitutive model applies to underground excavation soil, comparing with traditional loading mechanical model Duncan-Chang. Numerical simulation showed that the model unloading were closer to the engineering practice than conventional algorithms.