The invention relates to a manufacturing method of steel for a speed reducer gear of an electric automobile, belongs to the technical field of iron-based alloys, and particularly relates to steel for gears. According to the element composition design related, the steel comprises, by mass, 0.20%-0.35% of C, 0.05%-0.35% of Si, 1.20%-1.60% of Mn, 1.10%-1.50% of Cr, 0.20%-0.50% of Mo, smaller than or equal to 0.035% of S, smaller than or equal to 0.015% of P, smaller than or equal to 0.20% of Cu, smaller than or equal to 0.20% of Ni, 0.010%-0.080% of Al, 0.0060%-0.0260% of N and the balance Fe and inevitable impurities, wherein Al/N is equal to 1.5 to 3.9. The smelting technological process comprises the following steps of pre-treated molten iron and optimized scrap steel, primary smelting in an electric furnace, refining in an LF furnace, vacuum degassing in a VD furnace, continuous casting of square billets, shot blasting of the continuously-cast square billets, heating in a walking beam furnace, descaling with high-pressure water, rolling of the continuous casting billets, surface flaw detection and internal ultrasonic flaw detection of hot-rolled round steel, quality inspection and warehousing. The element components are matched with the smelting process, and the steel meets the requirements of high strength and long fatigue life of the steel for an electric automobile speed reducer. And remarkable advantages are obtained on the aspects of hardenability and grain uniformity.