The difference between the blood pressure of the left and right arm may be caused by inaccurate measurement, but it may also be caused by pathological diseases such as aortic coarctation, the former can be corrected by standardized measurement and multiple measurements, and most of the latter require timely medical treatment. The blood pressure of normal people’s right and left upper limbs is not exactly the same, but the difference is not more than 20mmHg, if the difference is too big more than 20mmHg, we should consider various reasons. 1. Inaccurate measurement, such as one side of the cuff tied too loose, one side of the sleeve is too thick, measurement of the position of the upper limb is not allowed, etc., through the standardization of measurement, repeated measurements can be corrected by the wrong values. 2. Pathological diseases, commonly seen in aortic valve stenosis, aortic coarctation, subclavian artery steal syndrome, arterial catheterization, arterial stenosis or occlusion of one side of the upper limb, and so on. It is necessary to further improve the ultrasound, magnetic resonance and other tests to clarify the diagnosis, and find the cause of the disease to give appropriate treatment. If the difference in blood pressure between the two upper limbs is large, it is recommended to go to the hospital.