Abstract: Reactor pressure vessels (RPVs) continue to age due to irradiation and environmental influences during long-term operation, which increases the risk of brittle fracture. Typically, structural integrity assessments of defected RPVs rely on deterministic evaluation methods. These methods assume the lower bound of fracture toughness and the upper bound of crack stress intensity factors, often combined with a safety factor, to produce conservative results. However, such deterministic approaches fail to account for the inherent randomness in critical factors, such as material properties and crack size, and therefore cannot fully reflect the reliability of defected RPVs. Therefore, evaluating the structural integrity and reliability of RPVs under various transient conditions is a key issue for the operation and life extension of nuclear power plants. This paper takes the RPV beltline of a domestic pressurized water reactor power station as the research object, and uses the probability analysis software FAVOR to conduct a probabilistic fracture evaluation and parameter sensitivity analysis of the RPV under pressurized thermal shock conditions and low-temperature overpressure conditions. A comparison of the reliability between the RPV of old power plants in the United States and the RPV of new domestic power plants under the same low-temperature overpressure conditions is presented. The results show that the through-wall crack frequency (TWCF) of the new domestic power plant RPV under selected typical transient conditions meeting the criterion of < 1×10⁻⁶/(reactor·year), and has a large safety margin. When the copper (Cu) content exceeds 0.3%, the average TWCF of the RPV fails to meet the required reliability index. Thus, it is recommended to select low Cu materials for RPVs or limit Cu content in control materials to no more than 0.2%. With the increase of defect depth and density magnification, the average TWCF rises by 3 to 6 orders of magnitude under most transient conditions. When considering the warm pre-stress effect, the average TWCF significantly decreases, with the average TWCF approaching zero under most transient conditions. A similar trend is observed for changes in the initial reference nil-ductility transition temperature, indicating that both the warm pre-stress effect and a lower reference initial nil-ductility transition temperature can effectively reduce the failure risk of the RPV. The RPV of new power plant has less Cu content, thicker cladding thickness, and no axial weld structure. These factors are the main reasons why the TWCF of the new power plant RPV under low-temperature overpressure conditions is much lower than that of the old power plant RPV.