This invention utilizes a simple addition operation to demodulate the structure of two bit differential fraction amplitude shift modulation receiving signals and a method, which regulates threshold value dynamically and the value can be used in judging if the amplitude of input signals is changed, in addition to that, it's only necessary to judge if the amplitude of an input signal is changed butnot needing to detect the variance rate of the amplitude exactly so as to simplify the complexity of hardware and increase performance of modulators.