Tunnel excavation in short distance will change the physical equilibrium stress field of stratum and cause adverse effects on adjacent existing pile. A simplified analytical solution to calculate the horizontal displacement of existing pile under the influence of unloading in adjacent tunnel excavation is presented. First, consider that the existing pile is a Euler-Bernoulli beam resting on the Pasternak foundation model. Then, the free ground settlement of existing pile caused by a modified Loganathan’s formula during shield tunnel excavation is considered. The differential equation of lateral displacement of existing pile is established by combining the displacement coupling condition of tunnel-soil-pile foundation. Finally, the analytical solution of lateral displacement of pile caused by adjacent tunnelling is obtained by finite difference method. The reliability of the analytical solution is validated by comparing with boundary element program, finite element simulation and field measurement. Then, results of different factors such as tunnel buried depth, tunnel excavation radius, clearance distance between tunnel and pile on existing pile is analyzed. Through parameter analysis, the theoretical formula of maximum horizontal displacement of pile caused by tunnel excavation is put forward. The research content can provide theoretical verification for the investigation on the influence of adjacent tunnelling on deformation of existing pile.