Abstract: With the scheduled back to back VSC-HVDC based unsynchronized interconnection between southwest power grid and central china power grid, southwest power grid frequency limit and the risk of instability increased. The current emergency control measures are mainly as follows, such as cutting off generator, cutting off load and DC power emergency support and coordination optimization. In this paper, the rapid adjustment of the output of hydropower units to participate in frequency coordinated control, was conducive to the flexibility and economy of frequency emergency control. Considering the frequency limit, the response time and the control cost of the frequency control measures, prioritizing the different frequency control measures. In the case of low-power surplus scenario,the rapid adjustment of hydropower units is the lowest cost. In the high-power surplus scenario, the adjustment time of hydropower units is about 10 s. Participating in steady-state frequency limit control, reducing the cost of additional cutting generator. Finally, through the actual grid simulation example,verified the effectiveness of the rapid adjustment of the output of the hydropower unit, and the rationality of the priority of the different emergency control measures.