The actual excavation process of the deep foundation pit is simulated by single sided lateral unloading true triaxial test by using rigid-flexible compound true triaxial apparatus.The stress-strain relation，breakage process and change characteristics of the soil strength parameters are analyzed under lateral unloading and vertical loading conditions during the excavation of the pit.Then the reduction factor is determined according to the change characteristics of the red clay strength parameters during the excavation and experience in engineering practice.The soil strength parameters are reduced with double strength reduction methods step by step.The mechanical process for the excavation of the red clay pit is simulated by using FLAC 3D procedures and Mohr-Coulomb model.The results show that the mechanical mechanism of the deformation and breakage process for red clay under lateral unloading condition is completely different from that of vertical loading.Soil strength parameters decrease while the depth of the excavation increases and the changes in range，magnitude and speed are changed incrementally.In the same vertical load condition，the soil under excavation condition is more deformable and easy to breakage compared to that under pressure conditions，which leads to the instability.