Xiaomen is a small island located at the edge of Houmen Channel, just off Xiyu Island (to which it is linked by a bridge). It is typical of the basalt mesa type of island and contains many of the natural and cultural features that characterize the archipelago as a whole. In this respect, it is a "Penghu in miniature."

On the northwestern coast of the island is Whale Cave, a large hole in an eroded columnar basalt formation that is said to resemble a whale when viewed from a distance. To the southeast of Whale Cave is a plateau covered with golden quartz sand. This is one of the few places in the Taiwan area where you can see a semi-arid landscape that has been eroded because of a lack of covering vegetation.