A theft warning system includes a first electronic device and a second electronic device. The first electronic device is configured for transmitting monitoring signals, wherein the frequency of the monitoring signals is capable of changing according to change in the voltage supplied to the first electronic device. The second electronic device communicates with the first electronic device by receiving the monitoring signals transmitted by the first electronic device, and generating alarms when a frequency of the received monitoring signals is changed. A theft warning method applied to the electronic device is also provided.