WepresentanewQuasi Λ model four levelsystembasedonageneralthree levelΛ modelandinvestigatetheresonantfluorescencespectroscopyindetail.Therearethreeultra narrowspectrallinesinthespontaneousemissionspectrafromupperleveltolower lyinglevelsofthetwoemissionchannels,whichoccuroverawiderangeoftheparameters.Theappearanceofthethreeultra narrowspectrallinesiscloselyrelatedwithRabifrequencyofthetwocoherencedrivingfields;andthelargertheRabifrequency ,thenarrowerthelines.Theeffectoflinenarrowingdoesn’toccurwhenthereisonlyacoherencedrivingfield .Whiletheincoherenttransitionandthecollisionaldecaybetweentheenergylevelsdestroythefeatureofthespectralnarrowingwhichmaybeconsideredtobetheresultofthemulti channelquantuminterference.