Abstract: The availability factor is a very important indicator to measure the design and operation performance of nuclear power units. It is also the economic performance indicator of WANO（The World Association of Nuclear Operators） to measure the world’s nuclear power plants. To improve the availability factor of nuclear power units is the focus and research direction of the industry. In this paper, on the basis of many years of good practices in CPR1000 nuclear power station, the availability model of nuclear power units is established, and the methods and ways to improve the design availability factor are provided, to optimize the design of new nuclear power plants and to provide the reference for the availability factor index evaluation in follow-up nuclear power plants.