Thispaperproposesanewqueuingmodelandadaptiveschedulingschemewhichrealizesmulti-classQoSmechanismunderDiffServarchitecture.Thequeuingmodeliscomposedoftwoparalleloutputsubqueues,eachoutputsubqueueadoptsrandomdropalgorithmbysettingdifferentbufferthresholdfordifferentclasstraffic,soitcanprovidemulticlassQoS.Thenewproposedschedulingschemewhichadaptivelychangestheparameterλcanguaranteetheperformancetargetofhighclasstraffic,inthemeantime,improvetheQoSoflowclassestraffic.