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The ion noise in microwave tube is studied using two-dimensional particle-in-cell method. The fluctuation of the ion amount, which is the source of ion noise of the tube, is observed in the simulation process. It is found that the sufficient ions amount in drift tube and beam envelope scalloping appear to be responsible for ion noise after changing the gas pressure, magnetic focusing field, and drift tube length. A great number ions escape radially and their amount is about half of the beam electron when ion noise occurs. The number of secondary electron is less than that of the ion and beam electron; it affects the ion noise very slightly. There are two ways can be used for depressing the ion noise. The first is adjusting or increasing the magnetic focusing field in drift tube. The second is creating static electric field gradient to make the ions leave the interaction space quickly.