With the further application of information technology in the digital control system (DCS) in nuclear power plants(NPPs) as the key infrastructure, the demand for endogenous cybersecurity in the process monitoring layer that plays an operational monitoring and management role in the DCS of NPPs is becoming increasingly prominent. This article first analyzes the endogenous cybersecurity requirements of the process monitoring layer in DCS and conducts technical research. Based on commercial cryptographic technology, a design of endogenous cybersecurity is proposed to achieve integrity and confidentiality protection of critical data storage and transmission. This design selects suitable commercial cryptographic algorithms and devices to implement storage and transmission security components. On this basis, a prototype was deployed and performance validation was conducted. The verification results indicate that this design has implemented the data storage and transmission intrinsic safety protection function of the process monitoring layer in DCS, meeting the performance requirements of the original system while having minimal impact on the original system.