Abstract: To reveal the roughness effect and shear load transfer mechanism of the pile-soil interface, this study proposed a roughness evaluation method for regularly grooved structural plates by comprehensively considering groove depth, groove width, and groove distribution. The interface peak shear strength was then decomposed into two components: the sliding friction strength over the smooth rib crest region and the shear strength of the soil within the groove region. A roughness effect coefficient η is introduced, and a predictive model for the interface peak shear strength was established. Furthermore, based on the disturbed state concept, a shear load transfer model for the pile-soil interface considering the roughness effect was developed. The results indicate that the proposed roughness evaluation method can better characterize the geometric differences of different groove parameters and reflect the relationship between roughness and interface shear strength. Under the present test conditions, a critical roughness value of Rcr=0.914 mm is identified. The proposed peak shear strength prediction model can reasonably predict the peak shear strength of pile-soil interfaces with different roughness values and normal stresses, with an average prediction error of 5.5%. The proposed shear load transfer model can effectively describe the nonlinear evolution of the shear stress-shear displacement relationship of the interface under the influence of roughness. The findings can provide a theoretical basis and reference for the quantitative evaluation of surface roughness, surface structure optimization design of threaded piles or ribbed piles, and the analysis of the bearing and deformation behavior.