Pile side friction is an important component of the bearing capacity of socketed piles, and plays a crucial role in improving the vertical bearing capacity of socketed piles. In order to study the load-bearing mechanism of socketed piles, on the study background of on-site monitoring of the bearing capacity of socketed piles of Qingchi Grand Bridge on Guiyang-Jinsha-Gulin Expressway, the development mechanism of pile side friction under construction loads is analyzed. Based on the shear characteristics of the pile-rock interface of socketed pile and theoretical analysis, the expressions of pile side friction and shear displacement are derived. A load transfer model of socketed pile is established based on the load transfer theory, and the model is solved by using the 4th-order Runge-Kutta method through Python. The on-site monitoring data of engineering piles are in accordance with the calculation result, indicating the correctness of the established load transfer model and the rationality of the calculation method, which can be used for study the mechanism of side friction of socketed piles. According to the monitoring data and the load transfer model, the characteristics of the axial force and shear displacement of the pile foundation section in the shear slip stage under different construction loads are analyzed. The result shows that (1) Under construction loads, the pile side friction of socketed pile plays a role from top to bottom gradually and bears most of the construction loads. (2) When the shear displacement reaches the ultimate shear displacement, the pile side friction remains stable gradually, and the construction loads are borne by the pile end resistance gradually. (3) The sectional axial force of socketed pile calculated by using the load transfer model is in accordance with the monitoring result, upon that the established load transfer model and calculation method have practical engineering significance. (4) The calculated ultimate pile end resistance is significantly smaller than that calculated according to the Design Specification for Highway Bridges and Culverts Foundations (JTG 3363—2019). This indicates that the original design of the ultimate pile end resistance is overly conservative and needs to be reasonably revised.