Objective：To discuss the dynamic changes of BNP in the patients of AMI and the clinical significance of the early diagnosis of AMI. Method：Plasma BNP in 22 SA cases，24 UA cases，20 healthy cases were tested and the plasma BNP of 41 AMI cases also were tested in 2，4，6 and 24 h after the onset of AMI.Result：BNP level of AMI group and UA group were obviously higher than that of SA group（P<0.01）.Plasma BNP level in 4，6，24 h after onset of AMI were obviously higher than that of UA group（P<0.01）.4 h and 6 h after AMI onset the surface area under ROC curve were 0.819 and 0.898. Conclusion：AMI patients have an increased plasma BNP level obviously 2 hours after the onset of AMI，and the BNP level increases significantly with the extension of tine. 4 h and 6 h after the onset of AMI the plasma BNP level have good diagnostic value for AMI.