It is pointed out that the electric power of a generation unit varies in sine wave with the rotator angle relative to the inertia center of the system. By using single machine equal area criterion （SGEAC） in combination with the time domain simulation method

the disturbed system is simulated for a very short period of time. Based on the simulation result

the curve of the electric power of a generator unit with its rotator angle relative to the inertia center of the system is fitted. Compared with the result obtained by using the real-time parameters of the generator and electrical network

it is unnecessary to simulate the disturbed system to the instant when the system loses its stability

in order to determine whether the system is stable or not. In comparison with the actual value

the error of the proposed method is in the range of ±5%. The physical meaning of the proposed method is clear.Practical system analysis shows that the method is very efficient

and it should have wide engineering applications for on-line stability monitoring.