Analyzing system trajectory from the perspective of individual machines provides a distinctive angle to analyze the transient stability of power systems. This two-paper series propose a direct-time-domain method that is based on the individual-machine equal area criterion. In the first paper, by examining the mapping between the trajectory and power-vs-angle curve of an individual machine, the stability property to characterize a critical machine is clarified. The mapping between the system trajectory and individual-machine equal area criterion is established. Furthermore, a unity principle between the individual-machine stability and system stability is proposed. It is proved that the instability of the system can be confirmed by finding any one unstable critical machine, thence, the transient stability of a multimachine system can be monitored in an individual-machine way in transient stability assessment.