Because 5Cr4W5Mo2V steel is characterized by high hardness, wear resistance, tempering resistance and thermal stability, its toughness is relatively low. 5Cr4W5Mo2V steel obtains lower bainite through different isothermal quenching processes, which has the characteristics of both strength and toughness, and improves toughness. By comparing the different microstructures and mechanical properties, the austempering process parameters were optimized. The results show that the lower bainite, martensite and retained austenite structures containing 60% lower bainite can be obtained when the steel is austenitized at 1150 ℃ and tempered at 280-330 ℃ for more than 60 min. After austempering for 120 minutes, the toughness of the steel increased significantly to 247.75 J/cm2.