Forafullychaotictwo-dimensional(2D)microcavitylaser,wepresentatheorythatguaranteesboththeexistenceofastablesingle-modelasingstateandthenonexistenceofastablemultimodelasingstate,undertheassumptionsthatthecavitysizeismuchlargerthanthewavelengthandtheexternalpumpingpowerissufficientlylarge.Itistheoreticallyshownthattheseuniversalspectralcharacteristicsarisefromthesynergisticeffectoftwodifferentkindsofnonlinearities:deformationofthecavityshapeandmodeinteractionduetoalasingmedium.OurtheoryisbasedonthelinearstabilityanalysisofstationarystatesfortheMaxwell–Blochequationsandaccountsforsingle-modelasingphenomenaobservedinrealandnumericalexperimentsoffullychaotic2Dmicrocavitylasers.