Abstract: The line-end coil insulation of inverter-fed machine is more prone to deteriorate due to the high-frequency (HF) pulse width modulation (PWM) switching transient overvoltage. The switching oscillation resonance effect has the potential to sensitively indicate the small insulation state change. However, the condition features are HF coupled by multi-factors. To address the problem, this paper proposes a novel feature extraction and decoupling method of line-end coil insulation using the high- frequency differential-mode (HFDM) switching oscillation. First, the multi-modal switching oscillation in the inverter-fed machine system is analyzed and the HFDM is selected as the sensitive mode to exclude the influence of distributed capacitance of inverter side power devices. Then, the variational mode decomposition (VMD) algorithm is employed to extract the HFDM frequency from multi-modal switching oscillation, and the effect of the dynamic switching time of power devices under different transient operating conditions is decoupled. Finally, the amplitude ratio of HFDM current between the inverter side and machine side is utilized to decouple the influence of cable HF parameters. Experimental study on a 380V/3kW permanent magnet motor drive system is conducted to verify the effectiveness. The results show that the proposed method can sensitively indicate the health state of line-end coil insulation. Moreover, the effects of an inverter, cable and operating conditions can be effectively decoupled.