Abstract: The steel with a ferrite + pearlite microstructure was quenched at 910 ℃+tempered at different temperatures (500, 550, 600 ℃) to obtain the ultra-high strength grade casing drilling steel. The obtainal casing drilling steel was subjected to impact tests at different temperatures (-60-20 ℃). The effects of tempering and impact test temperatures on the impact toughness and fracture mechanism of the casing drilling steel were studied. The results show that as the tempering temperature increased, and the martensite in the casing drilling steel gradually disappeared, and the tempered sorbite structure was formed. Meanwhile, the impact energy consumed in room temperature impact increased, and the maximum impact load decreased. The impact fracture macromorphology of the steel tempered at different temperatures was both fiber zone and shear lip, and the fracture mechanism was ductile fracture. The ductile-brittle transition temperature of the casing drilling steel tempered at 550 ℃ was -33.64 ℃. As the impact test temperature decreased, the impact energy gradually decreased, the fracture macromorphology changed from a complete fiber region to a nearly complete radiation region, and the fracture micromorphology changed from complete dimple to a quasi-cleavage structure containing local dimple structures.