In this paper, a series of monotonic triaxial tests were carried out at a relatively wide range of backpressure conditions (from 100 kPa to 29 MPa) to investigate its influence on the shear properties of silt sand.Backpressure effect on the shear strength, excess pore pressure, and volumetric strain behavior were analyzed,and the underlying influencing mechanism was preliminarily discussed. According to test results, the application of higher backpressure conditions on quasi-saturated samples resulted in a significant buildup of negative excess pore water pressure, which led to an increase in shear strength. The trend became less pronounced after the backpressure exceeded 5 MPa. Nevertheless, within this range, the shear strength increased by approximately 40 kPa for every 100 kPa increase in backpressure. Furthermore, the consolidated-drained (CD) test results revealed that the value of backpressure (at least within 29 MPa) had no detectable influence on the shear properties of saturated samples. It was indirectly proved that the undrained shear strength was influenced by the change of effective stress caused by excess pore water pressure in the shear dilation stage of quasi-saturated samples. The applicability of the effective stress principle to predict soil strength even in ultra-high backpressure conditions was confirmed.