Abstract: The seismic response characteristics of variable cross-section between main tunnel and fire engine access is numerically simulated by the finite element analysis software MIDAS GTS - NX to analysis the influence on the seismic response of variable cross-section of subsurface structure, background with a subway tunnel in Chengdu. The results show that under the effect of artificial seismic wave on the subway in Chengdu, the stress mainly concentrates on the joint of the upper concrete plate and the vertical support column. In the case of common earthquake, the main performance is the overall displacement of the structure, and the deformation is smaller. The maximum stress nodes of the cross section of an infrequent earthquake are likely to have a cracking phenomenon. When severe earthquake comes, the containment of subway tunnel turning from compressive stress to tensile stress, and the maximum stress value node is very close to the concrete tensile yield. Therefore, it is necessary to make reinforcement measures in the relatively large displacement node