The invention relates to a method for accurately calculating unoccupied parking spaces in a parking lot. According to the method, a parking space information unit, a detection unit and a server processing unit are provided; the parking space information unit comprises parking space sensing devices installed in each parking space; when sensing vehicles leaving the parking spaces, the parking space sensing devices transmit the vehicle departure information of the parking spaces, namely, parking space unoccupied information, to the server processing unit through the parking space information unit; when detecting that vehicles arrive at the entrance of the parking lot, the detection unit obtains vehicle arrival state information and transmits the vehicle arrival state information to the server processing unit; and the server processing unit puts the parking space unoccupied information and the vehicle arrival state information into statistics and obtains real-time unoccupied parking space information through calculation according to the total number of parking spaces of the parking lot. With the method of the invention adopted, the real-time information of the number of the unoccupied parking spaces of the parking lot can be obtained more accurately through statistics, and great convenience in parking can be brought to the owners of the vehicles, and time can be saved for the owners of the vehicles.