Based on dynamic thermal rating technologies and taking into account of voltage drop and steady-statestability limitation, the model and algorithm of online valuing loadability of transmission line under operation are established. Considering the generator units voltage regulation characteristics, the variable parameter dual-port Norton equivalent model based on expanded power ∞ow is derived, and the online tracking of the equivalent parameters of the system is implemented. Based on the historical samples, time series method is used to analyze the variation law of the equivalent parameters to make the maximum loadability of transmission line under operation closer to the actual situation. Studies have shown that the dual-port Norton equivalence can very well re∞ect the efiect of parallel ∞ow on the transmission line loadability in interconnected systems, and the changes in the equivalent parameters re∞ect the nonlinearity and time-varying properties of power systems, and by the equivalent parameters variation, the time-varying nature of the electromagnetic power is re∞ected. The feasibility and efiectiveness of the equivalent model and the algorithm are conflrmed by the analysis of the loadability of Weihai 220 kV transmission line under operation.