The　impacts　of　strain　evolution　on　transmission　dynamics　are　important　for　the　public　and　the　policymakers　during　the　spread　of　infectious　diseases.　A　varying-strain　model　is　proposed　to　analyze　the　small-scale　and　short-duration　epidemics.　First,　the　expression　of　basic　reproduction　number　is　obtained　by　next　generation　matrix　method.　Then,　the　stabilities　of　equilibrium　points　are　derived　by　Routh-Hurwitz　criterion　and　LaSalle＇s　invariant　principle.　Further,　combining　the　official　surveillance　data,　the　impacts　of　infection　scales　are　investigated.　Meanwhile,　the　sensitivity　analysis　for　basic　reproduction　number　and　peaks　of　infected　individuals　are　conducted.　The　results　of　this　study　provide　significant　insights　for　the　policymakers　controlling　small-scale　and　short-duration　epidemics.