In equipments running condition monitoring, the vibration analysis is the most used method, the analysis parameters of which include the amplitude characteristic and power spectrum. Different from the conventional regulation method, taking advantage of the phase characteristic of vibration signal, this paper deals with the equipments condition monitoring by the lag characteristic arising when the signal passing system. Here we carried on on-line monitoring to the condition of the vehicle hang system, and made a deep discussion on this issue from both theory and method sides. Compared with the analytical method based on the amplitude characteristic, the phase characteristic method is more effective. This new approach not only enriches the analytical contents of the vibration monitoring, but also broadens the analytical path of the vibration monitoring.