The invention discloses a method for promoting excess sludge to cultivate aerobic granular sludge by adding soapnut saponin. The method comprises the following steps of firstly, dissolving the soapnut saponin into an aqueous solution of the soapnut saponin; secondly, pretreating the excess sludge by using the aqueous solution of the soapnut saponin; thirdly, cultivating granular sludge in an SBR (Sequencing Batch Reactor); fourthly, settling and separating to obtain the aerobic granular sludge with the granular size of 0.1 to 3mm. According to the method disclosed by the invention, by adding the soapnut saponin in wastewater, the cultivation time of the granular sludge can be shortened; in addition, the solubility of organic matters which are indissolvable in water in a water body of high wastewater can be improved, the contact probability between microorganisms and a substrate is increased, the bioavailability is improved, an enzymatic reaction is improved and a degrading effect is enhanced. The method has high practical value for actual engineering application of the aerobic granular sludge, in particular to the aspect of treating industrial special wastewater with complex components.