Abstract: Q345 steel has been widely used in the field of rail transit due to its superior mechanical properties and cost-effectiveness. This article proposes a modified Paris curve method considering size effects to address the complexity of fatigue crack propagation behavior in key load-bearing components of rail vehicles during long-term service. Firstly, the crack propagation law of CT specimens of different sizes under standard loading conditions was studied through numerical fatigue tests; Secondly, the ligament connection length b was introduced into the traditional Paris curve model, and a quadratic function was used to fit the relationship between the material parameters lg C and b, constructing a modified formula considering size effects; Finally, the effectiveness of the correction curve was verified based on experimental data, with a goodness of fit R2 of 0.9867. Research has shown that the modified Paris curve can uniformly describe the crack propagation behavior of specimens of different sizes, significantly improving the accuracy of fatigue life prediction and providing reliable theoretical basis and technical support for the structural design and safety assessment of rail vehicles.