Abstract：A simplified expression under conventional three axis test conditions is presented after reviewing the constitutive relation of Drucker-Prager material with the parameters changing with internal variables. The axial plastic strains were selected as the internal variables and the relations between the increments of internal variables and the ones of external variables of strains and stresses were obtained. The values of internal variables at each point of recording in testing were thus obtained. In order to determine the values of two strength parameters corresponding to the same internal variable through a number of tests，the stresses of each test were interpolated among uniformly distributed values of internal variables；so that the strength parameters for a given internal variable can be derived from the yield surface equation. As such the variations of the strength parameters with the internal variables were obtained without the need of assuming one beforehand. Results of tests show that the variations of two strength parameters are not the same and the values of internal variables at peak stresses are different under different confining pressures，which agrees with the viewpoint of others that confining pressure can improve the ultimate strength and delay its coming.