Disclosed herein is a portable expansion module, an X-ray detector, and a method of controlling the X-ray detector. The portable expansion module includes a connecting portion configured to connect to a connector of an X-ray detector, and a display configured to operate using power supplied from the X-ray detector to the portable expansion module based on the connecting portion being connected to the connector, receive data from the X-ray detector based on the connecting portion being connected to the connector, and display state information of the X-ray detector based on the data received from the X-ray detector.