The effect of excavation-related unloading on the underlying soil will reduce the ultimate pile shaft resistance. In this study, Mindlin's stress solution was used to allow for the variation of vertical effective stresses induced by excavation around a single pile. The ultimate resistances of the pile shaft before and after the excavation were calculated. The results of theoretical calculation were compared with others through a case study. Furthermore, the variations of reduction factors for shaft resistance with excavation depth, excavation length, ratio of length to width and pile length were analyzed. The results showed that reduction factors for shaft resistance decreased firstly and then increased with the increase of excavation depth, and it decreased first and then tended to be stable with the increase of excavation length and ratio of length to width. With the increase of pile length, the reduction factors for shaft resistance increased.