Abstract: To ensure the safe operation of nuclear power plants, the status of the reaction pressure vessel needs to be monitored in real time. The fracture toughness of the domestic A508-Ⅲ steel, which is the main material, can be used as the primary indicator to reflect its condition. Recently, the size of specimens used for nuclear material testing and analysis is gradually decreasing, and this trend helps to improve the accuracy and efficiency of testing. The fracture behavior of mini-CT (1/6CT) specimens from a meso-scale perspective was studied in this paper, to reveal the fracture mechanism for a better understanding of the material properties. The uniaxial tensile testing of domestic A508-Ⅲ steel was carried out with the aim of gaining insight into this fracture behavior. Based on the testing results, the Ramberg-Osgood mechanical intrinsic model and the GTN (Gurson-Tvergaard-Needleman) meso-damage model, those can accurately describe the mechanical behavior of the material, were constructed. The model parameters were also calculated and determined based on the testing data, and the reliability of the models was verified by comparing the testing data with the simulation results. Subsequently, fracture toughness tests of 1/6CT specimens at different temperatures were also conducted to understand the effect of temperature on the fracture properties of the material by analyzing the morphological characteristics of the fracture. The results show that the width of the fracture toughness zone shrinks from 43 μm to 10 μm, and the average toughness size shrinks from 0.3 μm to 0.05 μm as the temperature decreases from -100 ℃ to -130 ℃. The plastic deformability of the material becomes more brittle as the temperature decreases. Based on the constructed mechanical intrinsic model and the GTN meso-damage model, the 1/6CT specimen was analyzed from crack sprouting and expansion to failure fracture using finite elements, and the results show that the J-R curve decreases with the decrease of temperature, and fracture toughness value (J_IC) and crack extension length (Δa) decrease by 43% and 3% respectively, with an error of no more than 5%. This indicates that the numerical method based on the GTN meso-damage model is a simple and effective way to study the fracture behavior of mini-CT specimens.