An illuminating device, an illuminating guidance system and an illuminating guidance method are provided. The method includes: detecting a vehicle entering a parking lot to generate vehicle entrance information by a detector; detecting at least one vacant parking space of the parking lot to generate vacant parking space information by a sensor; receiving the vehicle entrance information and the vacant parking space information to plan a guidance path by a controller; and controlling the LED light board to provide a guidance light pattern by the controller, thereby guiding the vehicle to the vacant parking space according to the guidance light pattern.