The invention relates to alloy steel for braking discs. Chemical compositions of the alloy steel comprise in percentage by weight: 0.21 to 0.26 percent of carbon, 0.8 to 1.0 percent of silicon, 0.5 to 1.0 percent of manganese, 1.2 to 1.55 percent of chromium, 1.95 to 1.99 percent of nickel, 0.34 to 0.41 percent of molybdenum, 0.1 to 0.2 percent of vanadium, 0.05 to 0.06 percent of niobium, less than 0.01 percent of tungsten, less than 0.02 percent of titanium, less than 0.02 percent of aluminum, 0.05 to 0.09 percent of copper, 0.02 to 0.03 percent of zirconium, and the balance of Fe and unavoidable impurities. The alloy steel provided by the invention is steel for casting, has higher mechanical strength and thermal fatigue resistance, also has better thermal cracking resistance, stable friction property and good thermal conductivity at the same time, and is particularly suitable for producing the braking discs used for rail transit vehicles such as city rail transit vehicles, railway locomotive vehicles, motor vehicles, high-speed trains, and the like.