Identifying and assessing threats is an essential prerequisite and key element for conducting risk assessments of nuclear industrial safety DCS control system. The risk assessment standard specification GB/T 20984–2022 provides principles for threat assessment and grading criteria, but does not include specific quantitative evaluation calculation methods. Meanwhile, there are almost no security threat samples in the operation and maintenance scenarios of nuclear industrial control safety DCS systems. Therefore, it is necessary to comprehensively evaluate the capability and frequency of threat generated by considering conditions such as the sources, types, and motives of threats. This paper identifies threats by combining the classification of threat sources and threat detection models such as CAPEC, targeting the operation and maintenance scenarios and key assets of nuclear energy industrial control safety DCS systems. First the overall research approach was determined, and through qualitative analysis and quantitative calculation of threat behavior capabilities and motives, ultimately forms a quantified threat assessment method applicable to the operation and maintenance scenarios of nuclear energy industrial control safety DCS systems. The research results show that this method can be effectively used for network security risk analysis and protection of nuclear industrial safety DCS control system.