Abstract: Tangent modulus was essential to nonlinear soil model,and it was necessary to understand it properly during numerical simulation.First,a new concept namely half strength index was introduced to build the mathematic properties equations of stress-strain relationship,then the deficiency of hyperbolic expression of the relationship was indicated in terms of it.Second,according to laboratory test data,differential control equation of soil stress-strain relationship was presented and a new 3-parameter stress-strain expression was established.It was shown that the hyperbolic expression was the simplified type of the 3-parameter expression.Third,an improved tangent modulus of soil was obtained based on the new stress-strain expression,and methods to determine its parameters were discussed.Finally,simulation of triaxial test data using the improved tangent modulus was conducted.