To explore the effect of excavation on the horizontal displacement of adjacent tunnels in soft soil area，firstly，the deformation mechanism of horizontal displacement of adjacent tunnels in soft soil area due to excavation is analyzed. Secondly，project cases on excavation of adjacent tunnels in soft soil area are collected. The importance of the factors affecting the maximum horizontal displacement of the tunnel is ranked by the random forest algorithm. Through analyzing the influencing factors statistically，a semi-empirical formula，which is relatively simple and convenient for engineering practitioners，is proposed. It can be directly used to predict the maximum horizontal displacement of the adjacent tunnel under excavation. The accuracy and applicability of the proposed empirical formula is verified by comparing with the measured data of the actual engineering cases published in the collected literature. Parameter analysis shows that: with the increase of excavation geometry，the maximum horizontal displacement of the tunnel increases in an approximate logarithmic relation，but the increasing speed gradually slows down. The horizontal displacement of tunnel is greatly affected by the horizontal displacement of retaining structure. It is approximate linearly correlated with the horizontal displacement of retaining structure. The distance between the excavation and the tunnel has a negative correlation with the maximum tunnel horizontal displacement. Furthermore，when the distance is less than twice the excavation depth，the influence of excavation on the tunnel is greater. Based on the proposed formula，the effect range of excavation on the tunnel is divided into zones，and the results can provide certain theoretical guidance for similar projects.