In order to clear the holistic safety of the underground combined structure under different fire scales, and provide reference for the fire protection design of tunnel structures, a nonlinear transient thermalmechanical coupling analysis model for the underground combined structure of Shangcheng Avenue Tunnel in Yiwu under fire is established by using finite element software ANSYS; the variation laws of the displacement and stress of the underground combined structure under different fire scales are studied; and the safety factor of each crosssection of the tunnel structure is analyzed as well. The results show that: （1） The increasing fire scale in the tunnel will deteriorate the mechanical properties of the concrete more seriously, and the displacement of the roof of the fireaffected structure will increase sharply under loading effect; nonetheless, the displacement of the roof of the adjacent horizontal combined structure decreases gradually, which will change the holistic deformation form of the combined structure; the stress redistribution occurs in the underground combined structure under fire, and the degree of changing stress will increase significantly with the increase of the fire scale. （2） When a megabus in the tunnel is ignited, the fireaffected structure has a greater impact on the stress and safety of the adjacent horizontal combined structure, but a smaller impact on the vertical combined structure, which still ensures the holistic safety of the underground combined structure.