Abstract: The research and application of the DC system as part of the security and stability control of the power grid has been limited to the preventive control and emergency control, which are referred to as the first and the second defense line. Fully exploring the characteristics and control capability of the DC system is an important technical way to improve the safe and reliable operation of an AC/DC power grid. The concept of using the emergency DC power support capability as the third defense line of a power system is proposed. The coordination and cooperation of the emergency DC power support with the under-frequency-load-shedding and the over-frequency- generation-tripping can be applied to prevent system collapses. When abnormally low/high frequency is detected, the emergency DC power support can be activated to reduce the amount of load shedding or generation tripping. A preliminary design is presented on the control parameter setting calculation and the abnormal frequency control device. The beneficial results of the emergency DC power support as a control measure of the third defense line is validated by simulations based on actual grid operation data.. The issues concerning the use of the proposed concept as the third defense line are discussed in relation to the timing and control characteristics of the DC system. The findings have importance reference value for expanded technical options for the third defense line to support safe and reliable operation of the power grid.