A sensor for sensing a usage status of an electrical device and its associated method are disclosed. The sensor is disposed at a near-end of a power line of the electrical device. The sensor provided with a magnetic detector, an analog-to-digital converter and a controller. The magnetic detector is close to the power line to detect a magnetic field change around the power line to correspondingly generate an analog signal. The analog-to-digital converter (ADC) is used to receive the analog signal and convert the analog signal to a digital signal. The controller is used to receive the digital signal and generate an indication signal to accordingly learn at least one of statuses of being turned on and turned off the electrical device.