Phase is a kind of important wave parameter of periodical signal. Making use of the combination of MCS-51 single-chip computer and external circuit and using its fiat resource fully, this paper has realized the high accuracy measure of phase difference by using the precision measurement technique such as over zero appraisal phase method, fill count by the pulse of high frequency and multiperiods etc. It also introducs systematic principle and the realization method of hardware and software emphatically.