In view of the excavation unloading characteristic, the variation of soilmass strength is studied through the theoretical deduction and the test analysis. Baseed on the Hvorslev’s real strength theory, the strength ratio of the unloading soil and the normal compressed soil considering the pore-water pressure is deduced and the test simulating excavation is carried out. Through comparing data of the theory and test, the soilmass is caused to be at the overconsolidated state, and the soil microstructure is damaged, then the soilmass strength is reduced in the unloading process. The analysis result of theory and test are helpful to the further understanding of the effect of unloading in excavation on the variation of the soilmass strength, which are very significant for avoiding project accidents.