Abstract: A modified constitutive model of saturated sands based on the phase transformation state can well describe stress-strain relationship of sands under undrained conditions. By improving the calculation method of the phase transformation state parameter in the model, an expression for e-lgp’ is modified. Through detailed elaboration, a relationship for e_pt-lgp is proposed and is verified through triaxial compression test data. The proposed new relationship is used to calculate the phase transformation void ratio e_pt of the sample under undrained condition, and then the phase transformation state parameters are calculated in the model. These calculated results from the improved undrained constitutive model fit well with undrained triaxial compression test results, verifying the effectiveness of the constitutive model and the rationality of the relationship for e_pt-lgp. Moreover the original boundary surface constitutive model can better describe the mechanical behavior of undrained, saturated sands under varying density and confining pressure conditions.