Background Worldwide, hypertension is an important public health challenge because of its high prevalence and the concomitant risks of cardiovascular disease. It induces half of the coronary heart disease and approximately two-thirds of the cerebrovascular disease burden. Vascular endothelial dysfunction has important roles in the pathophysiology of essential hypertension. Types I and II hypertension can be treated with sang-qi granules (SQG), a Chinese herbal formula. Several experimental studies on animals have shown that SQG can lower blood pressure and myocardial fibrosis by suppressing inflammatory responses. However, no standard clinical trial has confirmed this. Whether SQG can improve endothelial cell function is unknown. Methods/design In this randomized double-blind double-simulation controlled trial, 300 patients with stage I or II hypertension will be recruited and randomly allocated in a 1:1:1 ratio to group A (treatment with SQG and placebo instead of Losartan), group B (treatment with Losartan and placebo instead of SQG), and group C (treatment with SQG and Losartan). In this study, 10 g of SQG (or its placebo) will be administrated twice a day and 50 mg of Losartan (or its placebo) will be administrated once in the morning. The primary endpoint is the drug efficiency for each of the three groups. The secondary endpoints are the change in average systolic and diastolic blood pressure during the day and the night, the change in the rate at which blood pressure drops at night, assessment of target organ damage (heart rate variability, ankle-brachial pressure index, and pulse wave velocity), assessment of any improvement in symptoms (Hypertension Symptom Scale, syndrome integral scale in traditional Chinese medicine, Pittsburgh Sleep Quality Index Scale, Self-Rating Anxiety Scale, Self-Rating Depression Scale, and the 36-Item Short Form Health Survey), blood lipids, serum indicators of vascular function (changes in serum levels of ET-1, TXA2, NO, and PGI2), and safety indicators. Discussion This study aims to provide clinical evidence on the efficacy and safety of SQG in the treatment of hypertension. Moreover, the possible mechanism by which SQG may lower blood pressure will be explored by observing the protective effect of SQG on vascular endothelial function, as well as its effect on related clinical symptoms, risk factors, and the target organs of hypertension.