The invention discloses an electrode structure for a migration tube in a mobility spectrum instrument (IMS), comprising a circular electrode. At least one part of either circular surface of the circular electrode close to the inner margin is formed into a conical surface forming an angle different from that of a center shaft of the circular electrode. The electrode structure of the embodiment of the invention can effectively eliminate or reduce the accumulation of special charges in the migration tube and is particularly suitable for occasions when the strength of an electrical field in the migration tube is lower or a great amount of ions pass by the migration tube. Meanwhile, the outside diameter of the electrode can be greatly reduced in the structure while keeping the inside diameter of the migration tube unchanged so as to effectively reduce the outer size of the migration tube and achieve the purpose of reducing the size of the IMS.