Many of rock and soil materials exhibit strain softening behavior under load in geotechnical engineering practice. To show the strain softening properties of geotechnical material is crucial in the numerical simulation of geotechnical engineering. Based on previous researches, the authors analyze the circular cavity expansion problem which is one of the basic problems in geotechnical engineering, and compares the results of theoretical solution of strain softening model with FLAC^3D numerical calculations. The comparison confirms  the accuracy and applicability of strain softening model in FLAC^3D numerical calculation. Moreover, the numerical results obtained from computations by strain softening model and elasticperfectly plastic constitutive model are compared. Results show that the strength softening  of strain softening geotechnical materials could not afford to be ignored. Finally, the related parameters (ε^p_i and η ) of strain softening model are analyzed. The research results could help researchers understand the nature of mechanics in strain softening model of rocks in the numerical calculation.