It is a great challenge to keep frequency stable under the condition of large frequency difference in AC/DC hybrid terminal grid. At present, when it happens, the major measures include primary frequency modulation on the power side and under frequency load shedding on the load side. But now, DC power emergency support, units operating in a pumping condition and large-scale interruptible load can be controlled, and the technologies to control them can also be applied. It is possible to comprehensively upgrade and extend the emergency frequency control technology. This paper systematically analyzes the key technologies that need to be overcome to realize the whole process control of frequency emergency, and emphatically expounds technologies such as fault identification, accurate power loss calculation, multi-line coordination, coordination of many measures, system monitoring and management and so on. Based on the above key and core technologies, the Frequency Emergency Coordination Control System (FECCS) of east China power grid is constructed, which ensures the safe and stable operation of east China power grid and improves the consumption capacity of new energy. This work is supported by Science and Technology Project of State Grid Corporation of China “Research on Key Technologies of Close-loop Verification for Security and Stability Control System Functions and Reliability of District Grid”.