The invention relates to a method for extracting vanadium from vanadium-containing steel slag by using a highly-alkaline sodium hydroxide medium, belonging to the technical field of metallurgy. The technical scheme comprises the following steps: adding steel slag, water and NaOH into a reactor together, wherein the mass concentration of the NaOH solution is 65-90%; decomposing and leaching under atmospheric pressure, and diluting the obtained slurry with a diluter to obtain a slurry mixture; and separating the slurry mixture by filtering at constant temperature, and washing the slag with a washing solution to obtain tailings and a vanadium-containing water solution. The method provided by the invention can be implemented under atmospheric pressure at low temperature (greatly lower than the roasting temperature in the traditional pyrogenic process), and is easy to operate and safe; and the extraction rate of vanadium is greatly higher than that of the existing vanadium recovery technique based on the pyrogenic process and wet process, the one-time recovery rate of vanadium is 85-90%, and the total content of vanadium in the tailings is 0.3-0.6 wt%. The invention solves the problem that vanadium in the vanadium-containing steel slag can not be easily recovered, and has the advantages of high recovery rate of vanadium, low production cost, favorable economic benefit and environmental benefit, and the like.