Danhong injection (DHI) is the most prescribed injection form of Chinese medicine for the treatment of cardiovascular diseases such as atherosclerosis, angina pectoris, and stroke in China. However, its active components and action mechanisms remain poorly defined. We hypothesized that DHI contains active components that could prevent and restore endothelial dysfunction by improving vascular relaxation activity. DHI increased vasorelaxation in vivo and ex vivo of rat aortas. Vascular reactivity screen identified that danshensu was the major relaxation factor in DHI. DHI-mediated endothelial-dependent vasorelaxation was independent on nitric oxide/endothelial nitric oxide synthase but was via prostacyclin pathway by increasing cyclooxygenase (COX)-2 gene expression and prostacyclin production. Our results revealed a previously unknown endothelium-dependent vasorelaxation mechanism by danshensu and together with previously reported activity on ion channels of vascular smooth muscle cells, demonstrated that its dual actions contribute to a multicomponent Chinese herbal medicine that synergistically targets different pathways to achieve its well-documented cardiovascular protective effects.