In order to clarify the influence of the coupling effect of uneven temperature field and self-weight on the structural stress and stability of the assembled long-span steel-concrete composite beam bridge during the jacking construction process, the thermal-mechanical coupling analysis of the jacking construction stage of the long-span composite beam bridge under the influence of complex temperature field heat conduction was carried out. Firstly, the thermal-mechanical coupling analysis model of the most unfavorable incremental launching construction stage of a long-span steel box girder was established by ABAQUS. Then, on the basis of the validity of the model, the thermal coupling analysis under the influence of a vertical temperature gradient and heat conduction was carried out, and the distribution function of the temperature of the web along the height of the web at different times was obtained. Finally, the reliability of the analysis results of the heat conduction effect was verified by field measured data of different jacking stages. The results show that the stress of the main girder web is 275.7 MPa, and the relative error with the measured value is less than 5.8%. The vertical displacement of the guide beam reaches 0.73 m, and the relative error with the measured value is within 8.2%. The research results provide a theoretical basis for the safety and stability analysis of large-span steel-concrete composite beam jacking construction under the coupling effect of complex temperature fields and construction loads.