The static test of steel beam-columns was conducted to investigate the heat effect during process of welding strengthening while under initial load and the influence of different initial load on load-carrying behavior after welding reinforcement. The biaxial-symmetry I section columns were placed under boundary conditions that top of column was movable in plane while displacement constrained out plane and bottom of column was totally constrained. Therein 4 classes of initial loads were separately applied to the top of columns at a fixed in-plane eccentricity. Strengthening scheme was selected by symmetrically welding steel plates to outside surfaces of flanges, and grade of steel was carbon steel type Q345B. The displacement changes of specimen and welding stress and strain distribution of web were discussed during welding strengthening while under load, as well as the buckling mode and stability bearing capacity after strengthening. The results show that welding strengthening procedure decides the development mechanism of welding residual deformation and strain distribution. Initial load affects the magnitude of welding residual deformation while heat input and initial load coeffect the plateau length of load-displacement curve. Lower initial load has little influence on capacity while higher initial load obviously reduces capacity. Besides, the initial geometric imperfections also impact capacity after strengthening.