Abstract: With the changes of excitation and rotor speed, the amplitude/frequency of internal voltage in each generator is time-varying in the AC power system. The phasor method has been a classical method to analyze the sinusoidal steady- state circuits and has been widely used in power system analysis. First, with the integral relationship for the inductor between the instantaneous values in each phase, this paper describes the original series relationship composed of infinite terms between the power and the amplitude/frequency states of internal voltage for the transmission line. Then, the time-varying amplitude-frequency phasors sequence concept is proposed to analyze the formation mechanism and the influencing factors of the current and active/reactive power on inductor. The proposed new concept extends the phasor method of traditional sinusoidal steady-state circuit analysis to the dynamic process of AC power systems, and also uses the geometric description means of "phasor diagram". This method could also provide a reference in engineering for simplifying and using the original relationship between the terminal power and the states of internal voltage for the AC transmission line. Finally, the rationality of the method and conclusion can be verified by simulation.