The combined structure of tunnel, subway and utility tunnel is always the weak position in fire, thus it is vital to study its structure safety. Taking Yiwu Shangcheng Avenue tunnel as an example,  a fullscale fire model is established by finite element analysis software ANSYS, and the distribution law of structural temperature field under different fire scales is studied with thermal analysis function. Then the calculation method of ultimate bearing capacity of the roof of combined structure after fire is deduced by theoretical analysis. The results show that: (1) With the increase of fire scale, the radial influence range of concrete increases gradually, and the range of light, moderate and severe damage under different fire scales are defined; (2) The calculation formulas of bond strength between steel bar and concrete, compressive strength of concrete and yield tensile strength of steel bar under high temperature are proposed, and besides, the method for calculating roof ultimate bearing capacity of combined structure after fire is provided.