Abstract: The compact space inside a nuclear reactor greatly limits the arrangement of standard Charpy specimens for monitoring radiation embrittlement of reactor pressure vessels. The traditional standard specimens occupy a large amount of space in the irradiation supervision capsule, resulting in a shortage of irradiation supervision specimens. Although some have proposed sample miniaturization to improve the space utilization of irradiation monitoring capsules, providing sufficient data support for extending the lifespan of nuclear power plants. But this strategy brings a key challenge: there is a size effect on impact test datasets of different sizes, which cannot be directly compared and requires scaling the data through normalization methods. The establishment of normalization methods often relies on a large number of experiments as support, inevitably resulting in resource waste. As one of the core analysis methods in the current industrial field, finite element analysis is mainly reflected in multiple dimensions such as optimization design, risk assessment, and cost control. Therefore, this study focuses on the problem of specimen shortage caused by limited reactor space, and focuses on the core scientific issue of Charpy-V notch impact specimen size effect. Through uniaxial tensile testing and finite element inverse analysis, a Johnson Cook (J-C) constitutive model and failure model were established for ferritic/martensitic steel HT-9, and the model parameters were calibrated. Based on this model, correlations were constructed between impact test data of different sizes. At the same time, experimental evaluations were conducted on standard and KLST miniature Charpy-V notch specimens to obtain the effect of temperature on impact absorption energy. The data was fitted using a hyperbolic tangent function, and then the influence of size effect on HT-9 impact fracture behavior was systematically analyzed. The experimental results show that as the specimen size decreases, the ductile brittle transition temperature (DBTT) obtained from the Charpy impact test decreases, while the impact resistance of the material gradually increases with the decrease in size. The influence of three-dimensional constraints on the Charpy-V notch specimen and the effect on the upper shelf energy (USE) at different size scales were studied using numerical analysis methods. In addition, a normalized model was developed by fitting the finite element simulation results to correlate the usage of impact specimens of different sizes. Compared with the experimental values, the prediction relative error calculated by this conversion method is within 5%. And the effects of specimen thickness, relative notch depth, and notch root radius on the external constraint T_z and stress triaxiality h were investigated. Based on the variation law of equivalent plastic strain area of specimens of different sizes, a correlation was constructed between the ductile brittle transition temperature of standard specimens and KLST specimens. This research method demonstrates the accuracy and efficiency advantages of finite element based numerical simulation in solving the Charpy impact size effect problem, especially in linking specimen size with macroscopic fracture mechanics response.