The difference between left and right arm blood pressure is very big, if the difference is between 10~20mmHg, then it is normal; if it exceeds this range, then it is abnormal, which is commonly seen in multiple aortitis, aortic coarctation and so on. 1. Multiple aortitis: refers to chronic, multiple, non-specific inflammatory diseases of the aorta and its main branches. It can lead to narrowing or even occlusion of the arterial lumen. If it involves the main artery of the head and arm or one side of the subclavian artery, it can cause ischemia of one side of the upper limb, leading to a drop in blood pressure of the upper limb on that side, which causes a big difference in blood pressure between the right and left arm, and is usually accompanied by weakness and numbness of the limb, and other symptoms. 2. Aortic coarctation: It is a fatal disease in which the inner membrane of the aorta is torn due to various causes, and blood flows into the various layers of the aortic wall through the rupture of the inner membrane to form a coarctation hematoma. If it involves the main artery of the head and arm or the subclavian artery on one side, it can cause a decrease in the blood pressure of the upper limb on the affected side, resulting in a large difference in blood pressure between the left and right arms, usually accompanied by severe pain like tearing in the chest, chest tightness, dyspnea, coma and other symptoms. There are many other causes of a large difference in blood pressure between the right and left arms, which should be taken seriously, and timely consultation should be made to identify the cause of the disease, and further treatment or therapy should be prescribed by the doctor.