Abstract: Under significant disturbances, accurately identifying the key instability features of generator units and promptly computing quantitative transient stability indices are pivotal for ensuring grid stability. Utilizing real-time grid dynamic response data, this study constructs a mathematical model representing generator terminal voltage phasor trajectories in the complex plane, and examines their spatiotemporal characteristics and dynamic evolution. This analysis clarifies the critical steps involved in transient stability assessment. Subsequently, a method for identifying key transient angle stability indicators is introduced. The proposed method informs the development of criteria for transient stability identification and quantitative assessment metrics. This approach avoids the need for system equivalence, simplification, or identification of the coherent generator group, thereby addressing the shortcomings of existing response-trajectory-based methods—namely, their lack of quantitative information. Simulations validate the accuracy and effectiveness of the proposed methodology.