The invention provides an alloy steel and a manufacturing method thereof. The manufacturing method comprises the steps of: primarily smelting steel into molten steel; when the C content of the molten steel is 0.03-0.06 percent, the P content is not more than 0.015 percent and the S content is not more than 0.020 percent, tapping a steel ladle; during tapping, regulating the Si content of the molten steel to be 0.17-0.37 percent, Mn content to be 0.40-0.70 percent, Cr content to be 0.80-1.10 percent and Mo content to be 0.40-0.55 percent, and regulating the oxygen content of the molten steel to be below 0.0020 percent; controlling the Als content of the molten steel to be 0.02-0.0 percent, wherein Als represents acid-soluble aluminium; ensuring that the oxygen content is below 0.0010 percent when the molten steel is refined and the C content of the molten steel is 0.12-0.18 percent after the refining is ended in a steel ladle refining furnace; and carrying out circulating vacuum degassing treatment on the molten steel, and regulating the Ti content of the molten steel to be 0.006-0.010 percent to obtain the alloy steel. The alloy steel comprises the following components in percent by weight: 0.12-0.18 percent of C, 0.17-0.37 percent of Si, 0.40-0.70 percent of Mn, 0.80-1.10 percent of Cr, 0.40-0.55 percent of Mo, 0.006-0.010 percent of Ti, 0.008-0.015 percent of Als and the balance of Fe and unavoidable impurities.