For the problem of harmonic generation caused by power high frequency pulse during transmission, based on the transmission line theory, cable distributed parameter model under the high frequency was established and negative effects caused by long cable transmission were analyzed. Be tied of the states of three-phase AC-DC-AC PWM inverter with motor, not only the article discussed the transmission and reflectance processions of impulse wave over the cable in detail, but also researched the production principle of motor over voltage. In addition, it analyzed the reasons of premature failure of the stator winding insulations and why the supply generated harmonic. The theory of this method has been verified by the experiment.