The invention discloses a parking lot charging method, a background server, a road side unit and a system. Quite good reliability is realized. The method comprises steps that, when a target vehicle passes through an inlet of a parking lot, identity information of the target vehicle is acquired through video identification equipment, and the first present time is determined; if the target vehicle is equipped with a vehicle-mounted unit, the vehicle-mounted information in the vehicle-mounted unit is acquired; when the target vehicle passes through an outlet of the parking lot, the identity information of the target vehicle is acquired through the video identification equipment, and the second present time is determined; if the target vehicle is equipped with the vehicle-mounted unit, the vehicle-mounted information in the vehicle-mounted unit is acquired; if the identity information and the vehicle-mounted information are in matching with each other, corresponding fee is debited from the background system according to the first present time and the second present time, if the identity information and the vehicle-mounted information are not in matching with each other, an alarm signal is emitted.