During the construction of a new subway station beneath an existing operational tunnel, the shaft resistance of existing piles induced by excavation may decrease, and thus affect the bearing capacity of the piles. Currently, the theory of computing the shaft resistance for existing piles induced by excavation is not clear. Taking Shenzhen Metro Line 13 Gongming Square Station crossing the existing Line 6 project at zero distance as a case study, a calculation formula of pile shaft resistance induced by excavation under existing buildings is obtained by using the Mindlin stress solution. By comparison with the numerical simulation results, the correctness of the Mindlin stress solution is proved. The influence of parameter changes on pile shaft resistance and unloading stress is analyzed. The results show that: when the depth of excavation increases, the total pile shaft resistance decreases, and the ratio of sidewall unloading stress to total unloading stress increases; when the excavation area increases, the total pile shaft resistance increases, and the ratio of sidewall unloading stress to total unloading stress decreases. Therefore, when the excavation depth is large, the excavation area is small, or the shaft resistance of side piles is taken into consideration, the influence of the sidewall unloading stress must be taken into account.