The invention discloses a waste gas purifying method. The waste gas purifying method comprises the step of allowing oily fume waste gas to contact with a waste gas purifying agent, wherein the waste gas purifying agent comprises the following raw materials: sodium dodecylbenzenesulfonate, nonylphenol polyoxyethylene ether, isopropyl alcohol, triethanolamine, sodium silicate, citric acid, polyethermodified silicone oil, modified kapok fiber, nanometer alumina and water; the mass ratio of the polyether modified silicone oil to the modified kapok fiber to the nanometer alumina is (2-4): (5-10):(1-3); the modified kapok fiber is obtained by modifying kapok fiber with hydrogen peroxide; and the surface tension of the waste gas purifying agent is 18 to 32 dynes/cm. Compared with the prior art,the waste gas purifying agent provided by the invention is further improved in the maximum adsorption amount and adsorption rate of oil dirt.