Abstract: Taking the Main Steam Isolation Valve（MSIV） of the million kilowatt class pressurized water reactors in China as the examples, the design requirements and structure characteristics of MSIV were introduced in detail, and considering the present situation of the localization of MSIV, the technical difficulties and the key issues during the development were analyzed in-depth, such as the valve design and calculation, the driving device selection, quality assurance of materials, valve sealing performance, and valve prototype qualification. The relevant contents have been verified in the localization of MSIV.