In this paper,a series of submerged arc welding experiments were carried out to study the effect of interpass temperature on the impact toughness of 18MND5 steel welded joint for nuclear power. The results show that when the int-erpass temperature is below 250 ℃,the interpass temperature has little effect on the impact toughness of the weld. When the interpass temperature is above 250 ℃,the impact toughness of 18MND5 steel welded joint decreases as the interpass temperature increases. When the interpass temperature is over 350 ℃,the impact toughness of the welded joint reduces greatly. This further proves the rationality of strictly controlling the interpass temperature in the welding process required by relevant man-ufacturing standards and specifications.