By using real-time data of a large-scale receiving power grid， the impact of coordinated secondary voltage control（CSVC） on long-term voltage stability is assessed through time domain simulation. The results show that CSVC will help improve system voltage stability when the sub-regions have large adjustable reactive power margin， otherwise it will have little effect. Given that severe faults often result in reactive power shortage in receiving system， an emergency voltage control strategy is proposed based on trajectory sensitivity to pick up control generators according to real time system faults within the whole control range. Moreover， it can be easily combined with other control strategies to achieve better control performance. The time-domain simulation results prove that this emergency voltage control strategy can significantly improve system long-term voltage stability after large system disturbance.