Abstract: A series of undrained cyclic torsional shear tests and monotonic torsional shear tests are carried out on Fujian standard sand and Hutuo River fine sand under different initial static pore water pressures by using the hollow cylinder torsional shear apparatus in order to emphatically discuss the effects of the initial static pore water pressure on the development of the excess pore water pressure and undrained shear strength. The experimental results show that the initial static pore water pressure has a significant effect on the development of the excess pore water pressure, which affects the static and dynamic shear characteristics of sand. Specifically, during the undrained cyclic shear process, the greater initial static pore water pressure leads to the faster development of the excess pore water pressure and deformation. During the undrained monotonic shearing process, the greater the initial static pore water pressure, the greater the negative excess pore water pressure during the sand dilatancy, which significantly increases the strength of the sand. Based on the test results, the mechanism of the influences of the initial static pore water pressure on the excess pore water pressure and static and dynamic shear characteristics is preliminarily discussed. According to the results, the influences of the initial static pore water pressure on the anti-liquefaction strength of sand should be fully considered when we investigate the static and dynamic shear characteristics of soil (partially saturated soil) below the groundwater table, especially the liquefaction problem. In laboratory tests, the initial static pore water pressure (back pressure) should be determined according to the depth of underground water level where the sand is located.