In order to find out the mechanical effect of the construction of the cross-cutting tunnel backfill of the cross-cut interchange section of the full-section complex open-cut interchange, relying on the small clear distance interchange tunnel of the extension section of Jinzhou Avenue and Xingguang Avenue in Chongqing, the influence of different material backfilling, different tunnel section types and complex construction procedures on the mechanical characteristics of the underpass tunnel was analyzed by combining numerical simulation and on-site in-situ test, and the mechanical effect of the cross-cut interchange tunnel crossover crossover was determined.. The following findings are reached: 1) As the backfilling construction progresses, the displacement and internal force of the underpass tunnel grow gradually, with critical points of sudden change occurring at the end of the overlaying tunnel construction and the completion of the filling construction on both sides. 2) The stress changes in the arch waist and arch foot of the underpass tunnel are significant, and then, due to the replacement of C15 concrete at the interface, the upper and lower tunnels form a stable kidnapping system, effectively inhibiting the drastic change in stress at the arch waist and arch foot of the underpass tunnel. .3) The displacement and internal force of the underpass tunnel structure gradually expand from the intersection interface to the adjacent section along the longitudinal maximum, and once the construction is complete, the displacement and internal force peak value are transmitted from the intersection to the position of y=6m along the longitudinal adjacent section.