There is an ultra-low frequency oscillation and frequency instability risk faced by the Southwest Power Grid with its large proportion of hydropower and multi-DC outgoing. Thus a frequency stability control framework for this situation is constructed. Using traditional control measures, the functions of DC frequency control and emergency adjustment are introduced to improve the frequency stability defense capability of the outgoing power grid. Based on the analysis of the influence of a DC frequency controller on the frequency response characteristics of the outgoing power grid, the coordination relationship is considered between the DC frequency controller and primary frequency modulation and secondary frequency modulation considering DC power deviation. A multi-DC coordinated emergency power modulation scheme considering both frequency stability and power flow control after a fault is proposed. Given the actual operation of a AC/DC coordinated control system after actual fault, the effectiveness of the frequency stability coordinated control technology is discussed. This work is supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China (No. 5108-201926029A).