With the improvement of mechanical workpiece surface processing technology, the needs for real time and accurate measurement of high quality surface are increasing. In order to meet this demand, a new method of high accuracy, real time, non contact measurement of workpiece surface roughness is presented and a mathematical model of measuring surface roughness based on Laser Doppler effect is built. Laser Doppler technique is used to realize the measurement of nanometer displacement. Acousto optic modulation system is used to reduce the noise and raise the resolution of the measuring system. Through the optical path of optical heterodyne method can reduce the influence of outside interference on the measurement accuracy. The signal processing with high precision f/V conversion circuit and FFT algorithm is effective to improve the accuracy of the system. The experimental results show that the system for workpiece surface roughness measurement is feasible, and the precision can reach to 1‰. This shows that the system not only can be used to measure surface roughness but also can be used to calibrate other equipment.