The invention relates to a differential frequency circuit of plesio-digital signals and a differential frequency method thereof. The two plesio-digital signals are respectively connected with two input ends of a first mixer of the differential frequency circuit and also respectively connected with two input ends of a second mixer thereof by a phase-shift circuit of the differential frequency circuit. The differential frequency circuit of the plesio-digital signals and the differential frequency method thereof are technically characterized in that: the output ends of the two mixers are respectively connected to the two input ends of an adder; the output ends of the adder are connected with a filter and a shaping circuit in sequence; the differential frequency method conducts frequency-mixing processing on the two plesio-digital signals by the two mixers; the adder conducts analog adding processing for the mixed signals; and after output of the adder is processed by filtering and shaping, the adder outputs square wave signals with duty cycle of being close to 50 percent and uniform cycle. By adopting the adder behind the two mixers, the differential frequency circuit of the plesio-digital signals and the differential frequency method thereof remove parts of harmonic synthesis items which are close to the frequency of differential frequency in output of the mixers, thus providing a foundation for the measurement of high resolution and high accuracy of a plesio-digital signal output type sensor.