Abstract: The excavation and unloading of a foundation pit will lead to the deformation and destruction of the adjacent tunnels. In engineering, the isolation piles are often set between the foundation pit and the tunnel to protect the tunnel. In order to study the horizontal displacement laws of the existing tunnels outside the isolation piles under the influences of excavation of foundation pits, a three-dimensional mechanical model is established. Considering the stratum loss caused by excavation of foundation pits, the formula for the additional stress caused by unloading of the foundation pits is derived. Based on the Kerr foundation model and the adjustment of the foundation coefficient, the formula for the deflection deformation of the isolation piles is obtained. Based on the formation loss in the affected area, the formula for calculating the horizontal displacement of the existing tunnels is deduced. At the same time, the case is verified and the influencing factors are analyzed. The "traction effects" and "protection effects" of the isolation piles are explained theoretically. The results show that the calculated results are consistent with the measured data and numerical results. Setting isolation piles can effectively reduce the horizontal displacement of the tunnels and play a good protective role. The protective effects of the short piles gradually increase with the increase of the length of the isolated piles. After the pile length reaches the critical point, the "traction effects" of the isolation piles occur, and the protective effects decrease with the increase of the pile length. The protection effects are better when the pile spacing is small. When the isolation piles are placed close to the side of the foundation pit, the protection effects are significantly better than those of the side close to the tunnels.