In the test of deformation characteristics of soft clay soil in Shanghai, the test of deformation and strength of undisturbed soil under different stress paths is rarely carried out in engineering investigation, which makes the parameters provided in the investigation report not well consistent with the actual soil stratum in the excavation and unloading process of foundation pit. In this paper, taking a deep foundation pit project in Xujiahui, Shanghai for example, K0 stress path triaxial test was performed in the investigation stage. The test included loading, unloading and then reloading of the selected representative soil samples, and recording the stress and strain as well as modulus of deformation at every stage. The analysis of the test results shows that the stress path had an obvious effect on the soft clay soil, and the K0 stress path can reflect the deformation characteristics of the soil in the process of foundation pit excavation in the natural state. It is suggested to strengthen the promotion and application of the K0 stress path in the deep and large foundation pit projects which are sensitive to deformation.