The invention discloses a control method and system for avoiding sustained low EHV DC power. The control method comprises: calculating a commutation bus voltage harmonic distortion rate; determining whether AC voltage distortion is caused by excitation inrush current according to the voltage harmonic distortion rate; determining whether the excitation inrush current is in a downward trend according to the voltage harmonic distortion rate; and when the AC voltage distortion is caused by the excitation inrush current and the excitation inrush current is in the downward trend, reducing the output value of a commutation failure predictive function. The method and system avoid continuous low EHV DC power due to the excitation inrush current, not only support small-scale modification of a program, but also have universal applicability.