During the construction of a new subway station beneath an existing running tunnel, the shaft resistance of the existing pile induced by the excavation may decrease and the bearing capacity of the pile may be affected. Presently, the computation theory of the shaft resistance for the existing pile induced by the excavation is not clear. Taking Shenzhen Metro Line 13 Gongming Square Station to cross the existing Line 6 project at zero distance as the background, the calculation formula of shaft resistance of pile induced by excavation under existing building is obtained by using the Mindlin stress solution. By comparing with the numerical simulation results, the correctness of the Mindlin stress solution is proved. The influence of parameter changes on shaft resistance of pile and unloading stress is analyzed. The results show that: when the depth of excavation increases, the total shaft resistance of pile decreases, and the ratio of side wall unloading stress to total unloading stress increases; when the excavation area increases, the total shaft resistance of pile increases, the ratio of side wall unloading stress to total unloading stress decreases. Therefore, when the excavation depth is deep, the excavation area is small, or the shaft resistance of side pile is considered, the influence of the side wall unloading stress needs to be considered.