Abstract: The calculation method of the stress intensity factor (SIF) caused by thermal loading and pressure at inner corner of nozzle was studied by using 3D finite element method (FEM) with real crack in this paper. Taking a nozzle inner corner of reactor pressure vessel (RPV) in a project as an example, the SIF caused by thermal loading and pressure was calculated through 3D FEM with real crack and simplified engineering calculation method currently used, and the results of two methods were compared and analyzed. The result shows that if the SIF calculated by the simplified engineering calculation method is closed to the limit, the FEM should be used for the SIF of thermal loading in order to avoid the fast fracture risk caused by the non-conservativeness of simplified engineering calculation method.