Abstract: By studying the principle of characteristic impedance mismatch of uniform transmission line, it is found that the characteristic impedance mismatch of transmission cable will lead to the distortion of excitation current amplitude of load terminal. The key parameters of main circuit for power supply are analyzed, the amplitude and rise time of the excitation current at the magnet are decreased, when the matching impedance of the uniform transmission line is mismatched. By establishing the equivalent model of barrier point in uniform transmission line, the reflection coefficient at the barrier point of the uniform transmission line is derived. Based on the analysis of series resistance and parallel resistance at the barrier, it is found that the mismatch of transmission line characteristic impedance will lead to the decrease of active power at matching load. It is verified by the breakdown of high voltage cable.