Abstract: In order to assess the biaxial or multi-axial fatigue behavior of some particular structure details of steel bridges, the stress field intensity method was applied to establish a field diameter solving model of steel bridge components. Based on that, the concept of effective field diameter was put forward by reference to the railway steel bridge design code, and an analysis model for solving the biaxial fatigue strength was built. Taking the steel bridge gusset plate as example, its fatigue strength can be calculated through the field diameter solving model, in an error of less than 15%. Furthermore, the biaxial fatigue strength of the steel bridge gusset plate was predicted by the analysis model for solving the biaxial fatigue strength, and the quantitative relation among the input loads in each direction was obtained. The results show that to keep the same fatigue life, when the load in the main direction increases, the load in the secondary direction should decrease in a magnitude -0.5 times less than the load increment in the main direction.