Series capacitor has become an effective way of governance DC Bias, this paper elaborates the theoretical and simulation analysis of its principles and system over-voltage it caused. In the paper, firstly, it made ??a simple analysis of the principle of DC Biasing; then established the appropriate model with a typical substation in Yunnan Power Grid, the simulation analysis and actual measurement data comparison proved that the model accurately and efficiently; and then simulation of DC distribution of related substation was done, followed by series different capacitor in the neutral point ,it provided a theoretical basis for the capacitor in series with the selection; at last, the overvoltage, especially neutral overvoltage was made in detail, the results show that, series capacitor method may affect transformer neutral point over-voltage to a certain extent. Therefore, to selecting an appropriate capacitor, you need to consider the inhibitory effect of the neutral point of over-voltage and bias DC.