It has shown that the fatigue damage caused by thermal load cannot be ignored. The fatigue stress distribution of flat steel box girder bridge is greatly complex due to its complicated structural characteristics. In order to calculate thermal-vehicle coupling fatigue damage of flat steel box girder bridge, the thermal gradient fatigue model should be established. In this paper, This paper takes a large-span cable-stayed bridge as an engineering background. The long-term monitoring of spatial temperature field for this bridge was carried out. The simplified temperature stress solution formula for flat steel box girder sections is reduced, the difference of daily extremes was utilized for stress cycle statistics, and equivalent temperature stresses were calculated based on the damage equivalence principle. The relationship between temperature stress and temperature gradient was analyzed, and the equivalent temperature calculation method was proposed. Finally, a temperature fatigue loading model for flat steel box girders is proposed, which is a folded line model with representative temperature values of 4.5°C, 18.4°C and 14.1°C, respectively.