An alternating current detector with automatic judging of the amount of element under testing and monitoring, comprising a detecting input unit, a first output level unit, a second delay unit, a second output level unit and a cascade next element unit. The detecting input unit comprises a first delay unit and a first inverter. The first delay unit receives and delays an input signal. The input terminal of the first inverter is connected to the first delay unit and inverts the input signal. The first output level unit comprises a first NAND gate, a first diode and first resistor. The input terminal of the first NAND gate is connected to the alternating current signal of an element, another input terminal of the first NAND gate is connected to the output terminal of the first inverter.