The normal area of the mitral valve should be 4cm-6cm, when the mitral valve is attacked by rheumatism, the area of the valve will gradually shrink over time. 1.5cm-2.0cm is mild stenosis; 1.0cm-1.5cm is moderate stenosis; less than 1.0cm is severe stenosis. The basic lesions are thickening of the mitral valve leaflets, adhesions, fusion, and contracture of the subvalvular tendon cords, which result in a smaller opening of the mitral valve orifice, restricted opening, and even obstruction. Therefore, the normal area of the mitral valve is usually 4cm-6cm, so that it can perform its normal function. If the stenosis is mild, it usually does not affect too much, but if it is moderate or severe stenosis, it needs to be treated. Treatment may include balloon dilatation or direct mitral valve replacement. It is very important to have a normal area of the mitral valve orifice, which is 4cm-6cm.