The invention relates to a two-step method tempering process for improving impact toughness of a nuclear power large forging. The process includes the following steps that (1) the quenched nuclear power large forging is heated to 300-450 DEG C for heat preservation; and (2) the forging is heated to 600-660 DEG C continuously for heat preservation, then the forging is cooled in a furnace, and the process is completed. Compared with the prior art, the impact absorbing energy average value of steel treated by the two-step method tempering process can reach about 100J. The impact toughness is obviously improved compared with the nuclear power large forging treated by a traditional tempering process. Moreover, the process is simple, easy to operate and implement and the like.