Random energiziation of unloaded transformers may generate inrush current. In order to solve this problem, controlled switching technology is used for suppressing the inrush current. Based on the analysis of the principle of controlled switching of unloaded transformers, the changing laws of magnetic flux undercircumstances of simultaneously, rapidly and delayed closing strategies are studied, and the applicable conditions of the three closing strategies are compared and analyzed. Finally, the test platform for controlled switching of 10 kV unloaded transformer is built and the inrush currents under different closing phases are analyzed when residual flux is zero or not. Tests results show that closing at the voltage peak can effectively reduce the inrush currents when the core has no residual flux; but inrush current still can be generated when the core has residual flux. Therefore, it is necessary to estimate the residual flux in order to obtain the optimal closing phase and realize the no impact transition of energizing unload transformer. This work is supported by National Natural Science Foundation of China (No. 51477142) and Science and Technology Project of State Grid Corporation of China (No. 5229CG15003Q).