Novel Zeeman birefringence dual frequency laser(DFL) may produce dual frequency laser with tunable frequency difference from 3 to 40 MHz.The interferometer employing this laser as light resource will have top measurement speed of 4 m/s or even higher.The two lights are linear polarization lights with perpendicular polarizing directions.Systematic optic path is designed and constructed according to this character,and ideal frequency shifting is observed as the result.The past heterodyne signals processing methods may not be planted to use on mid-frequency difference.Big number minus method is improved and become suitable for processing mid-frequency heterodyne signals,and the system may count without cumulate error when input frequency is 50 MHz or below.