This　paper　proposes　an　optimal　resource　allocation　method.　The　method　is　to　maximize　the　Energy　Efficiency　(EE)　for　an　Energy　Harvesting　(EH)　enabled　underlay　Cognitive　Radio　(CR)　network.　First,　we　assumed　the　Secondary　Users　(SUs)　can　harvest　energy　from　the　surrounding　Radio　Frequency　(RF)　signals.　Then,　we　modelled　the　EE　maximisation　problem　as　a　joint　time　and　power　optimization　model.　Next,　the　optimal　EH　time　allocation　factor　can　be　calculated.　After　that　the　optimal　power　allocation　strategy　can　be　obtain　by　the　fractional　programming　and　Lagrange　multiplier　method.　Finally　simulation　results　show　that　the　proposed　iterative　method　can　be　better　performance　advantages　compared　with　the　exhaustive　method　and　genetic　algorithm.　And　the　EE　of　this　system　model　is　significantly　improved　compared　to　the　EE　model　without　considering　EH.