Contactless power supply for rail transit vehicles has good prospects for development. Accurate detection of vehicle position to guide power supply is a key techinique for contactless power supply. Traditional photoelectric induction and electromagnetic position detection methods have some defects, such as low detection accuracy and large external interference. Since the equivalent impedance of coil will change before and after the coil enters the ground coil . A method of transmitting narrow pulse to the front-end inverter of transmitting coil for contactless power supply vehicle is proposed, In this way, the current of the primary coil can be detected to determine whether the vehicle enters the ground coil or not. The measured current changes are used to judge whether the ground coil is fully put into operation or not. Finally, the feasibility and validity of this method are verified by experiments.