Abstract: The reliability and the usability of mechanical products largely depend on their surface roughness. In order to achieve long-distance measurement of surface roughness of mechanical parts, a method of electronic direct modulation laser coherence based on laser Doppler effects was put forward to measure the surface roughness. The application of laser Doppler effects can reduce influence of working distance. The speed of measurement was improved and the cost was reduced by using electronic direct modulation method. By using coherent method, alternative phase lock amplification was realized and the system noise was effectively controlled too. The experimental results show that the uncertainty degree of the measurement system can be reached 1 after controlling the statistical error and calibrating the experiment data during measurement process. The study has a certain guiding significance on the measurement of surface roughness.