Thevenin equivalent model which is based on the real-time measurement information provides a new researching method for on-line estimating static stability limit of transmission channel in multi-machines system. In according to the characteristics of the power system operation state, and the hypothesis that during a period of time the Thevenin equivalent potential and the impedance are unchanged, a new Thevenin equivalent model is proposed. And the Thevenin equivalent parameters can be estimated by means of recursive least squares with variable forgetting factor. In this way the transmission channel can be equivalent to a dual system with improved Thevenin equivalent method. So we can get the static stability limits of the transmission channel rapidly with the dual system. A method to monitor static stability of multi-machine system is also given in this paper by adopting the idea of calculating transmission channel static stability limit. The IEEE-39 system is used as a testing system. And the simulation results demonstrate that the method proposed in this paper can get the Thevenin equivalent parameters precisely and assess the steady-state stability of the transmission channel rapidly.