Nowadays, most power utilities design the emergency control schemes based on a large number of offline simulations, since the online measurement-based emergency control methods are far from mature and reliable at this moment. This paper proposes a novel measurement-simulation (M-S) hybrid method for transient stability assessment (TSA) and the emergency control scheme. Based on the deviation between offline simulation trajectory and the online trajectory equivalent to a single-machine-infinite-bus (SMIB) system, a new index named Deviation Energy (DE) is defined to fast assess the transient stability of the system online. This novel TSA method does not require trajectory prediction and can fully take into account local controls. Further, an additional control scheme is proposed to correspondingly stabilize the system, if the system is judged as unstable. The proposed method has been tested on the IEEE 39-bus system and an actual interconnected power grid model.