The invention discloses a polishing agent, method and system for polishing an inner runner of a 3D-printed nickel base alloy workpiece. The method comprises the following steps that the polishing agent containing phosphoric acid, nitric acid, hydrochloric acid, hydrofluoric acid, ferric chloride hexahydrate and sulfosalicylic acid is prepared; the polishing agent is heated to 40-60 DEG C and then conveyed into the inner runner of the workpiece and keeps flowing at a certain speed so that the inner runner can be polished; and the polishing agent is discharged, and the workpiece is washed. Through the polishing agent, method and system for polishing the inner runner of the 3D-printed nickel base alloy workpiece, the surface smoothness of the inner runner of the 3D-printed nickel base alloy workpiece can be effectively improved, so that the inner runner becomes uniform and smooth; and compared with traditional polishing methods for the inner runner of the 3D-printed nickel base alloy workpiece, the polishing agent, the method and the system are better in polishing effect, more convenient and quicker.