Abstract: Several forms of temperature distribution on cross section of bridge structures and their mechanical influence on structures are introduced.According to the basic theory of two-dimensional unstable temperature field,considerig the third condition to solve the differential equation of heat conduction as temperature load,the sunshine-induced time-varying temperature distribution on the cross section of a real bridge's steel-concrete composite beam was simulated using ANSYS.The thermodynamic properties of different materials in composite beams were evaluated.Comparing the in-situ measured temperature values with those from calculation,the proposed method is verified,and the reliability of the definition about the temperature distribution on cross section in Chinese code for bridge design is also proved.