Abstract: The microstructure and toughness of the heat affected zone of 1 000 MPa grade ultra-high strength hydropower steel were studied by Gleeble thermal simulation. The results show that the microstructure of the heat affected zone of 1 000 MPa grade ultra-high strength hydropower steel was mainly composed of martensite and bainite, and there were more small angle grain boundaries at peak temperatures of 1 320 ℃ and 850 ℃. At peak temperatures of 1 320 ℃, the impact absorbed energy was only 35 J, the crack formation energy was 28 J, and the crack propagation energy was 7 J. The impact absorbed energy was mainly composed of crack formation energy. When the peak temperature was less than 1 100 ℃, and the impact absorbed energy was greater than 100 J. The impact absorbed energy was mainly composed of crack propagation energy. At the peak temperature of 1 320 ℃, the toughness of the material increased first and then decreased with the increase of t_8/5. When t_8/5 was more than 40 s, the carbon element diffused fully and the carbide particles aggregated and grew up, which promoted the formation and expansion of cracks. The coarse grain zone of the ultra-high strength hydropower steel joint was the weak area of the whole joint. The heat input of ultra-high strength hydropower steel was recommended to be 15-60 kJ/cm.