A wearable device and a method for performing a registration process in the wearable device are provided. The wearable device includes a light source, a light sensor and a microcontroller that performs the method. In the method, the light source is activated to emit a detection light and the light sensor senses a reflected light. A light intensity of the reflected light is calculated. Specifically, an upper limit and a lower limit are referred to for detecting whether the wearable device is properly worn by a person. For example, since the wearable device can be worn on the person's wrist, the registration value is used to detect whether the wearable device is away from the wrist.