Transient stability detection technologies based on wide-area response are important means for real-time warning of transient instability. Based on the changing track of oscillation center voltage, a transient instability warning strategy for interconnected power grid is proposed. Firstly, the transient instability criterion based on ucosϕ trajectory integral is improved to overcome the relative lag of instability detection under some complex faults. Secondly, a ucosϕ trajectory penetration identification algorithm is proposed to form the ucosϕ composite criterion without increasing measurements, which further improves the reliability of instability detection. Finally, combined with the transient instability characteristics of China Southern Power Grid (CSG), a scheme of regional transient instability warning is designed. The transient stability is evaluated in real time by synthesizing wide-area and multi-station information. Simulation results show that the proposed method can identify the transient instability more quickly and accurately, and it would provide basis for emergency control after extreme contingencies.