A fall sensor coupled to a necklace to detect a fall of a user is provided. The fall sensor includes a first sensing device, a second sensing device, a wireless module and a controller. The first sensing device is coupled to the necklace to detect a tension force of the necklace. The controller is coupled to the first sensing device, the second sensing device and the wireless module. When the first sensing device detects the tension force is changed, the controller activates the second sensing device to confirm whether the user is falling down, and when the user is determined to be fallen down, and the controller outputs an emergency message via the wireless module.
