To investigate the fatigue performance of Q420qENH welded joints after corrosion, electrolytic accelerated corrosion tests and fatigue experiments were conducted on specimens, combined with numerical simulation. A thermal-mechanical coupled finite element model was established to simulate the welding process. The fatigue life of corroded welded joints was predicted using ABAQUS and FE-SAFE by introducing residual stress fields and stress concentration factors. The results show that all specimens fractured at the corrosion pits in the middle of the weld toe. Residual stress presents tensile stress, and the fatigue life of specimens decreases significantly with the increase of fatigue stress amplitude and corrosion time. The relative errors between test and simulation results are within 15%. The S-N curves (95% survival rate) fitted from test and simulation fatigue data show that the maximum relative error at 2 million cycles is 4.31%, indicating small errors and good prediction accuracy. According to the Code for Design of Steel Structures of Railway Bridges (TB/T 10091-2017), it is recommended to adjust the fatigue allowable stress amplitude category of butt joints for railway non-painted Q420qENH weathering steel to Category Ⅶ in bridge design.