In order to improve the performance of bus-bar protection, the probable influence of the neutral line impedance of AC secondary circuit on the bus-bar differential protection is analyzed theoretically by investigating a mal-operation of bus-bar differential protection. The analysis shows when a fault occurs on the light-load or unload circuit connected to bus-bar and the non-sinusoidal periodic current reaches to a certain level, the current transformer of sound phase could be saturated reversely because of the neutral line impedance, which can produce an abnormal current in the sound phase and thus cause mal-operation of bus-bar differential protection. The EMTP simulation results validate the correctness and rationality of the analysis. Some appropriate preventive measures are put forward to avoid such mal-operation.