Abstract: In order to simulate the strength changes of foundation caused by different building load, the traditional method of shear test is improved. Accurate additional force (beyond the capacity of force of soil covered above the undisturbed soil) is imposed on the sample in advance by using the automatic pressure consolidometer, and consolidated quick shear tests are carried out. Two kinds of unsaturated undisturbed soils are selected as study objects (one is mucky silt and the other is silt with silty sand) to analyze the influence on shear strength indexes according to the changes of their structures and states. Beginning with the relationship between stress and settlement of shear soil and shear box, this article employs the equation to calculate the dynamic shear area to derive the correction formula of shear normal stress, which is used to calculate the shear strength indexes. The results show that different types of soil have different properties. The changes in shear strength of the mucky silt are mainly in terms of cohesiveness. For the silt and silty sand, the angle of internal friction is the main factor. Water content and void ratio have the inverse relationship with the shear strength indexes.