Abstract: Using acoustic emission technology to monitor the martensite transformation process of 45 steel. The acoustic emission signal of martensite phase transformation was collected under continuous cooling condition, and according to the information entropy theory, the martensite phase transformation temperature of 45 steel was determined. The result show that the martensite phase transformation starting temperature of 45 steel determined using information entropy theory was 335.7 ℃, and the error rate was 1.26% comparing with the actual phase transformation temperature.