The American nuclear power plants achieved good economic benefits after the United States applied the maintenance rule, and the technology transfer documents of AP1000 nuclear power introduced from the United States also have the contents related to the maintenance rule. This article introduces the content and implementation of the maintenance rule, emphasis on the first step of maintenance rule “Selecting the scope”. Take the chemical and volume control system (CVCS) of a new nuclear power plant, for example, first, we analyze the components of CVCS if they are safety-related, if they are, the components belong to the scope of the maintenance rule. If the component is non-safety-related, we will analyze if it is relied upon to mitigate accidents or transients or is used in plant emergency operating procedures (EOPs), or the failure of it could prevent safety-related structures, systems, and components (SSCs) from fulfilling their safety-related function, or the failure of it could cause a reactor scram or actuation of a safety-related system. The component will belong to the maintenance rule if it meets one of these. We list the components of the CVCS that belong to the maintenance rule by this way. The study of selecting the scope will lay a foundation for introducing the maintenance rule into the nuclear power plant and carrying out the maintenance rule next step in China.