The invention discloses a signal loss detection circuit and method and an amplifier. The amplifier comprises a peak detector, a reference threshold generator and a comparator, wherein the peak detector is used for detecting the peak of an input signal and outputting the peak of the input signal to the comparator; the reference threshold generator is used for generating a judgment threshold and sending the judgment threshold to the comparator; and the comparator is used for comparing the peak of the input signal with the judgment threshold and giving out signal loss alarm when the peak of the input signal is less than the judgment threshold. According to the invention, the detection on loss of signal (LOS) is realized by virtue of the peak detector and the comparator, so that the design complexity is reduced greatly, the circuit scale and the circuit complexity are reduced, and the power consumption is very low.