Abstract: The effect of loading rate on the safety of engineering structures is one of the important considerations for their safety assessment. This paper takes A508-III steel, a nuclear pressure vessel material, as the research object, and adopts the improved split Hopkinson pressure bar system to carry out three-point bending (SEB) fracture toughness tests on A508-III steel specimens of different sizes under different loading rates, and evaluates the fracture behavior of A508-III steel under dynamic loading through dynamic fracture toughness J_Id and K_Id. The results show that the dynamic fracture toughness of A508-III steel increases with the increase of loading rate, and the effect of geometry on the dynamic fracture toughness of A508-III steel is weaker than that of loading rate. The results have practical value for engineering structural design and safety assessment, and provide an important reference for the structural design of reactor pressure vessels, which can guide the material selection and structural design of nuclear pressure vessels, and thus improve the safety and reliability of nuclear engineering.