Considering the issue of reasonably selecting the strength index in different construction procedures of high-speed railway cut slope, borehole shear test (BST) was carried out on silty soil. The influence of loading addition way on in situ strength characteristics of silty soil was studied by adding different normal stress levels and unloading to a certain stress after adding a higher normal stress (herein called preloading), taking Hajia high speed railway as engineering background. The results show that in-situ stress-strain curves of silty soil generally present the variation characteristic of weak hardening. Preloading greatly contributes to the shear strength, mainly in cohesion. With the increase of preloading from 0 kPa to 400 kPa, cohesion increases distinctly in the increment of 18.7~51.8 kPa, while internal friction angle rarely varies. Both cohesion and its increment after the preloading are nearly negatively linear to the water content of silty soil, but a critical preloading may occur when water content reaches to a certain value. The cohesion is positively linear to preloading, which should be fully used to back calculate the required preloading according to the safety factor that the project should satisfy. It provides a reference to engineering design and construction.