Abstract: This paper introduces a laser ranging system suitable for the medium and long range. The phase method is adopted to calculate the measured distance according to the phase information carried by reflected signal. The method of frequency difference of phase finding and the preprocessing of automatic gain control are conducted. As a result, the calculated phase difference is changed from high frequency to low frequency, improving the stability of phase difference measurement and reducing the ranging error. In data processing, the maximum likelihood estimation method is used to process the signal. The DDS and PLL mixing in the outside of loop systems contribute to more accurate results. The system has the advantages of high precision, no touch, good quality and affordable price.