The　bit　error　in　quantum　communication　is　mainly　caused　by　eavesdropping　and　noise.　However,　most　quantum　communication　protocols　only　take　eavesdropping　into　consideration　and　ignore　the　result　of　noise,　making　the　inaccuracy　situations　in　detecting　the　eavesdropper.　To　analyze　the　security　of　the　quantum　E91　protocol　presented　by　Ekert　in　collective-rotation　noise　channel,　an　excellent　model　of　noise　analysis　is　proposed.　The　increment　of　the　qubits　error　rate　(ber)　is　used　to　detect　eavesdropping.　In　our　analysis,　eavesdropper　(Eve)　can　maximally　get　about　50%　of　the　key　from　the　communication　when　the　noise　level　approximates　to　0.5.　The　results　show　that　in　the　collective-rotation　noise　environment,　E91　protocol　is　secure　and　the　raw　key　is　available　just　as　we　have　knew　and　proved.　We　also　presented　a　new　idea　in　analyzing　the　protocol　security　in　noise　channel.