In order to study the mechanical performance of screw anchor under vertical compression load in silty clay area, firstly, based on the static load test of screw anchor, the differences of methods for determining its ultimate bearing capacity at home and abroad are discussed respectively; secondly, a refined finite element model is established for screw anchor, in which the compression effect of screw anchor on surrounding soil is considered, and the numerical analysis results and test results are compared; finally, based on the verified finite element model, the change rule of anchor internal force and side friction resistance along the range of foundation depth under different load levels is analyzed, as well as the proportion relationship between side resistance and anchor end resistance bearing load. The results show that the lgP-s method is suitable to determine the ultimate bearing capacity of the screw anchor foundation with large anchor pieces; the friction between the screw anchor and the soil increases with the increase of the down-force displacement, and the friction near the anchor plate fluctuates greatly due to the deformation of the anchor plate; the proportion of load shared by anchor pieces accounts for more than 75% of the load of the screw anchor foundation, the maximum difference of load shared by each anchor piece is about 6%, and the end resistance of screw anchor foundation can be negligible.