Azo dye is a common pollutant，and many researchers focus on its treatment. In this study，the ultrasonic assisted ball milling technology was used to degradate the Congo red. With the combination of ultrasonic cavitation，effect of zero valence iron and mechanical chemistry，the hydroxyl radical concentration increased significantly in solution. Then，reaction speed was apparently accelerated under the oxidation of hydroxyl radical and the reaction of nano iron. Eventually，the removal rate of Congo red reached 99% within 2 minutes. Most of the organic matters were mineralized，and the removal rate of total carbon (TOC) reached 80% with 10 min. The degradation efficiency increased significantly and the degradation effect was more thorough under high concentration. It is a cost-effective，simple and environmental method. All these indicated the ultrasonic assisted ball milling process is valuable to be applied to the degradation of azo dye wastewater.