Abstract: P91 steel was subjected to 8-layer 16-pass submerged arc welding at different interpass temperatures (250, 300 ℃), and the weld metal was post-weld heated at 760 ℃ for 2 h at cooling rates of 55 ℃ · h⁻¹ and about 20 ℃ · h⁻¹ (furnace cooling). The effects of interpass temperature and post-weld heat treatment cooling rate on the impact toughness and corrosion resistance of weld metal were studied. The results show that when the interpass temperature was 250 ℃, with post-weld heat treatment cooling rate increasing from about 20 ℃ · h⁻¹ to 55 ℃ · h⁻¹, the carbide content and its aggregation degree at grain boundaries of the weld metal decreased, the impact absorbed energy obviously increased by about 68%, and its fluctuation degree obviously decreased; the free-corrosion potential increased, and the free-corrosion current density decreased. When the post-welding heat treatment cooling rate was about 20 ℃ · h⁻¹, with the interpass temperature increasing from 250 ℃ to 300 ℃, the carbide content and its aggregation degree decreased, the impact absorbed energy slightly increased, and its fluctuation degree slightly decreased; the free-corrosion potential increased, and the free-corrosion current density decreased. Within the range of test parameters, the interpass temperature and the post-weld heat treatment cooling rate of submerged arc welding of P91 steel should be controlled at 250 ℃,55 ℃ · h⁻¹, respectively; at this time the weld metal had the largest impact absorbed energy and free-corrosion potential and the smallest fluctuation of impact absorbed energy and free-corrosion current density, and the impact toughness and corrosion resistance were the best.