The inrush current that is several or even a dozen times of rated current can appear when the transformer closing switch without loads. This current flow through one side of transformer, So, it drains into the current differential loop and may lead to the false action of protection. The simulation models are set up by means of the Matlab/Simulink software according to the actual parameters of the 220kV new generation smart substation"s transformer in Hunan power grid. The inrush current under different closing angle is simulated and calculated. The Fast Fourier Transformation algorithm(FFT) is used to calculate the harmonic content of inrush current, also, the generating mechanism, discontinuous angle are also researched and analyzed. The rationality of protection setting value can be verified with this method according to the research results.