In the UV-LIGA fabrication process of a microinjection metal mold,the large internal stress of a SU-8 layer can result in SU-8 mould defects such as micro cracks, distortion or even exfoliation. In order to avoid these defects, the ultrasonic stress relief technology was introduced into the microinjection process. First, the SU-8 mould was obtained by coating, soft baking, ultraviolet exposure and post-exposure baking,and it was treated by the ultrasonic stress relief equipment before developing. Then the backless plate growing method was adopted to directly fabricate the nickel micro-electroforming pattern on a 38CrNiMnMo mould steel substrate and the SU-8 layer defects emerged in the process such as the distortion and exfoliation in the SU-8 spin coating on a non-circular substrate, the air bubbles in photoresist layer and the adhesion of plating layer with the substrate were eliminated or improved. Finally, the microinjection mold with a channel width of 80 μm and a height of 35 μm was fabricated. The results show that by using the ultrasonic stress relief technology, the SU-8 mould defects caused by large internal stress in SU-8 layer are overcome, the capacity of manufacturing microinjection mold via UV-LIGA technology is enhanced, and the success rate of microinjection mold fabrication is greatly improved.