Normally, the blood pressures in the two upper arms are not equal, and the blood pressure in the right arm is usually higher than that in the left arm. This is because the right arm reflects the blood pressure in the right brachial artery, and the left arm reflects the blood pressure in the left brachial artery. The right brachial artery comes from the innominate artery, the first major branch of the aortic arch, and the left brachial artery comes from the left subclavian artery, the third major branch of the aorta. The first major branch is closer to the main branch, so it has a higher pressure, so the blood pressure is higher on the right side than on the left. Under normal circumstances the difference does not exceed 20mmHg, if the difference in blood pressure between the two upper arms exceeds 20mmHg, adequate attention should be paid. In clinical practice, the following diseases should be excluded, including subclavian artery developmental abnormality, aortic constriction, supravalvular aortic orifice stenosis, aortic entrapment aneurysm, aortic arch syndrome, and polyarteritis.