In order to investigate the influence of reinforcement welding heat effect and different initial load on load-carrying behavior of I section steel beam-columns, three specimens strengthened by welding while under different load were simulated. Based on the thermal-structural coupling method considering heat effect, the analysis improved thermal input of heat source model and also considered initial geometric imperfection, initial residual stress and friction. Time history of displacement during welding, web stress-strain redistribution and load-displacement relationship were discussed, and meanwhile the effectiveness of the finite element analyses(FEA) were verified by comparing the FEA results and test results. Furthermore, welding temperature field, stress-strain redistribution between flange and strengthening plate and flange margin yield capacity, which could not be measured by test, were acquired. And by comparison of bearing capacity results and code calculation results, the present design methods were discussed. The results showed that welding strengthening procedure decided the development of welding residual deformation, while thermal input and initial load effected the range of displacement change during welding under load and magnitude of welding residual deformation. Higher initial load propelled the development of stress-strain redistribution toward the eccentric loading direction, thus leading to lower bearing capacity. However, initial residual stress had little effect on ultimate capacity. FEA method considering heat effect showed certain practicability and overall security, and code design method remained space for improvement still.