The invention relates to steel for a steam-temperature ultra-supercritical thermal power unit and a preparation method thereof, belonging to the technical field of heat resistant steel. The steel comprises the following chemical compositions by weight percentage: 0.06-0.10 percent of carbon, 0.1-0.5 percent of silicon, 0.2-0.8 percent of manganese, not more than 0.004 percent of phosphorous, not more than 0.002 percent of sulfur, 8.0-9.5 percent of chromium, 2.5-3.5 percent of tungsten, 2.5-3.5 percent of cobalt, 0.03-0.07 percent of niobium, 0.10-0.30 percent of vanadium, 0.80-1.20 percent of copper, 0.006-0.010 percent of nitrogen, 0.010-0.016 percent of boron, 0.01-0.04 percent of rare-earth cerium, not more than 0.01 percent of nickel, not more than 0.005 percent of aluminum, not more than 0.01 percent of titanium, not more than 0.01 percent of zirconium and the balance of ferrum and inevitable impurity elements. The steel has the advantages that the steel can be used for preparing G115 steel of 650 DEG C of steam-parameter ultra-supercritical thermal power unit and a large-diameter boiler tube thereof, and the room-temperature mechanical property, the impact property, the high-temperature mechanical property and the permanence property of the steel are much higher than those of P92 steel in the Gb5310 and ASME standard.