A wearable device comprises a body (1) and an outer casing (2) detachably mounted on the body (1). The body (1) is provided therein with a first printed circuit board (PCB) and a body antenna connected to each other, and the first PCB is provided thereon with a control circuit. The outer casing (2) is provided thereon with an outer casing antenna (201), and when the outer casing (2) and the body (1) are assembled, the outer casing antenna (201) is connected to the first PCB. The control circuit controls switching between the outer casing antenna (201) and the body antenna to use the outer casing antenna (201) or the body antenna as a working antenna. Also provided are an outer casing and a method for controlling an antenna of a wearable device.