How to maintain the design basis to maintain or improve the safety level of nuclear power plants in their lifetime, the major nuclear power operation countries led by the United States and France have made a beneficial attempt. There is no systematic research and Application on the maintenance of design basis in China. With the accumulation of operation time of domestic nuclear power plants, we will soon face the challenge of design basis and the resulting pressure of safe operation. Design basis maintenance concept was raised from international Nuclear Power Plants (NPPs) operating experience, due to the safety and operating performance being challenged by the missing or violating of design bases. Investigation in the international experience of design basis maintenance is carried out and basis and key performance factors are identified base on the research in World Association of Nuclear Operators (WANO) and International Atomic Energy Agency (IAEA) suggestion and criteria. Methodology framework is proposed based on the basis. The framework clarifies the goal and tasks of design basis maintenance, raises a systematic method that carrying out margin management during NPP daily operation and maintenance, controlling the modification and carrying out periodic safety review to maintain the design bases. It can provide a useful reference for how to carry out design basis maintenance in operational nuclear power plants.