The design of a external sealing structures is of much importance for the miniaturization design of valve. The metal diaphragm structure is the main component of the external sealing structure. So the mechanical behaviors of the metal diaphragm are significantly necessary to be determined for the valve design. This paper, by numerical simulations, studies the mechanical behaviors of the diaphragm, including: i) the stresses and deformations of a three-layered metal diaphragm under the loads of axial force and high pressure, and ii) the effects of the diaphragm layer number on the axial force and the maximum equivalent stress under effective external sealing. These results provide guidelines for design and application of the valve and its diaphragm.