In order to achieve smooth wet etching surface of silicon, ultrasonic technology is introduced in the wet etching of silicon. By improving the ultrasonic wet etching system, the temperature difference between top and bottom etching solutions can reach 0.5℃. Then, at 60℃(100), silicon is etched wetlly by KOH solution in mass ratio of 10%, ultrasonic frequency of 59 kHz and ultrasonic power ranging from 60 W to 180 W (every 10 W). Finally, the post-etched surface roughness is measured by Laser Scanning Confocal Microscope (LSCM), and the effect of ultrasonic parameters on the quality of etching surface is discussed. Experimental results indicate that the smooth etching surface can be obtained in roughness Rq of 0.02 μm at ultrasonic power of 120 W. The quality of etching surface is greatly improved in the ultrasonic wet etching system, also the etching surface of high quality can be obtained with suitable ultrasonic parameters in lower temperature and concentration of KOH solution.