Abstract: The ductile-brittle transition of pressure vessel steels caused by the change of loading rate and geometric constraint is a key issue that needs to be solved in the field of nuclear energy safety. In order to accurately analyze the dynamic fracture behavior of Chinese A508-III steel, the fracture toughness tests of Chinese A508-III steel under different loading rate and geometric size were carried out by using INSTRON VHS high-speed material testing machine to investigate the effect of loading rate and geometric size on the dynamic fracture toughness of Chinese A508-III steel. The results show that the Chinese A508-III steel has good impact toughness. With the increase of loading rate, the total impact absorbed energy of the specimen basically remains constant, the absorbed energy of crack initiation increases, while the absorbed energy of crack propagation decreases. The J-Δa resistance curve and the conditional initiation toughness J_Q decrease with the increase of geometric constraint, but increase with increasing loading rate. When loading rate reaches a critical value, the conditional initiation toughness J_Q basically keeps constant and the fracture mode of the specimen changes from ductile fracture to ductile-brittle-ductile mixed fracture. The maximum J-integral value J_max is more suitable to describe the fracture toughness evolution of Chinese A508-III steel when brittle fracture occurs due to the occurrence of mixed fracture mode. With the decrease of out-of-plane geometric constraints, J_max increases linearly with the increase of Δa_m. The higher the in-plane geometric constraint of the specimen has, the greater the slope of the linear relation between J_max and Δa_m presents. As the geometric constraint of the specimen increases, the ductile-brittle transition loading rate of the material increases, but the value of J_max decreases. Changing the geometric constraint can only change the fracture mode of the material within a finite loading rate range, and when the loading rate exceeds a certain threshold value, the loading rate becomes the main factor affecting the fracture mode of the material.