The present invention provides a parking space sensor, which comprises a sensing module, a control unit, and a trigger module. The sensing module senses the status of a parking space and generates a sensing signal. When the parking space sensor, and hence the trigger module, is flipped over, the trigger module triggers the control unit to perform an initialization procedure, in which the control unit transmits an initialization signal to the sensing module for initializing the sensing module. Thereby, the status of the parking space can be sensed and a sensing signal can be generated. Then the control unit produces a piece of parking space information according to the sensing signal.