Abstract: The transformation of microstructure and hardness with the extension of aging time on the 17-4PH Martensitestainless steel at 350℃ is studied in this paper, and the change of dynamic fracture toughness and fractographys of the stainless steel for various holding time at this temperature are also studied by instrumental impact test and scanning electron microscope. The results indicate that the crack initiation energy （E_i）, crack propagation energy （E_p）, absorbed-in-fracture energy （E_t） and dynamic fracture toughness （K_1d ）of this type of alloy Charpy v-notch sample is decreased with the continuation of time at 350℃, it means that the toughness of the alloy is degraded, and the hardness of the steel is ascended when aging time is expanded and reaches the maximum at 9000h. The fractography of this steel changes from dimple fracture into cleavage fracture and inter-granular rapture.