A power level identification circuit includes a power interface, a voltage conversion unit, a waveform detection unit, a signal pre-processing unit and a processing unit. The power interface receives electrical power and first signals indicating a level of the power. The voltage conversion unit receives the power from the power interface and converts the power. The waveform detection unit filters the power and obtains the first signals indicating power level. The signal pre-processing unit shapes the waveform of the first signals indicating power level and thereby generates second signals indicating power level. The processing unit identifies a level of the second signals indicating power level and switches to one of modes of the power level identification circuit corresponding to the identified level of the second signals indicating power level. A power level flag circuit and a power level supply system are also provided.