Cognitive　radio　technologies　are　commonly　considered　as　an　effective　way　to　solve　the　problem　that　currently　spectrum　resource　is　almost　used　up.　In　this　paper,　by　introducing　the　concept　of　small　cells,　we　propose　an　optimal　dynamic　spectrum　management　scheme.　To　allocate　the　spare　spectrum　resources　with　the　optimization　objective,　the　system　throughput,　power　consumption　and　the　interference　of　users　and　other　small　cells　are　taken　into　account.　Dynamic　spectrum　management　is　modeled　as　a　restless　bandits　model,　which　has　the　indexability　feature.　To　simplify　the　optimization　problem,　utilizing　the　indexability　feature,　simply　selecting　the　cell　with　the　smallest　index　value　guarantees　the　optimality.　Simulation　results　show　that　comparing　with　the　existing　methods,　the　optimal　spectrum　allocation　method　can　significantly　improve　the　system　reward.