Abstract: In view of the problem that transmission performance of wireless power transmission system is deteriorated with change of coupling distance in over-coupled region and in order to achieve the maximum power output with fixed-frequency mode, a new method was proposed which achieves impedance dynamic compensation by changing the generalized coupled factor variable of two planar spiral coils in primary coil circuit, and the mechanism of impedance dynamic compensation strategy were studied. Firstly, taking E-type double-tube LLC resonant inverter circuit as research object, circuit theory was used to establish model and analyze system output power, transmission efficiency and interaction mechanism of characteristic parameters; the variation law of system output power curve and transmission efficiency curve of the system were obtained, and it is concluded that the generalized coupling factor between two compensation coils has a direct impact on the system output power curve and transmission efficiency curve. Secondly, the mechanism of impedance dynamic compensation strategy was revealed, and an implementation program with fixed-frequency mode was presented. In the end, the correctness and validity of the theoretical analysis were verified by experiments.