Abstract: High temperature δ ferrite in the friction stir welding (FSW) joint of reduced activation ferrite/martensitic (RAFM) steel is the main factor affecting its impact toughness. This paper combined spray cooling to decreases the peak temperature of welding thermal cycles and provide rapid cooling of the joint to inhibit δ-ferrite formation. Fluid simulation software Fluent was used to simulate the temperature field for FSW of RAFM steel under different working conditions of spray cooling, and the integrated simulation results were verified experimentally. The results show that liquid nitrogen-assisted friction stir welding (FSW + LN₂) reduce the peak temperature of the welded joint effectively and accelerate the cooling rate of welding temperature reduction. The impact toughness of FSW + LN₂ joint improves from 23 J to 33 J, which matches equal toughness of the base material. Besides this, the change tendency of hardness is basically consistent with that of conventional FSW joints, the microhardness of the welded joint is much higher than that of the base material.