The cyclic weakening mechanism of pile-soil interfaces can affect the bearing capacity of piles. This paper investigated the cyclic weakening mechanism of constant normal stiffness in pile-soil interface under different shear-displacement amplitudes and initial normal stress parameters. An independently developed large-scale constant normal stiffness shear device is used in this study. The results show that the shear stress-displacement curve of the interface develops in a "closed loop" shape. The shear stress continues to weaken when the number of cycles increase. The weakening mainly occurs in the initial stage of shearing. This is mainly due to the rearrangement and breakage of soil particles at the pile-soil interface， as well as decompression and softening at the interface. These findings also indicate that the greater the initial normal stress leads to larger shear-displacement amplitude. Furthermore， the increase in number of cycles results in a rapid weakening of the shear stress. The logarithmic weakening can be used to predict shear stress under different test conditions. The results show that nonlinear logarithmic curve fitting has about 90% similarity with the real data curve， which is significant in studying the weakening mechanism of pile-soil interfaces.