The invention relates to a five-axis NC (numerical control) milling method for the internal surfaces of bent pipes, particularly relates to a method for machining the internal surfaces of bent pipes by using a five-axis NC milling machine, belonging to the field of NC processing. The method is implemented based on UG software, and used for machining the internal surfaces of bent pipe components by using five-axis NC machining equipment and lengthened ball cutters. The method comprises the following steps that: before the internal surfaces of bent pipe components are machined, the operations of carrying out division on processing regions, carrying out planning on cutter paths, and carrying out interference processing on cutter shafts and workpieces are performed; a clamping and positioning operation is performed on processed bent pipe components; and the cutting operation of the cutter in the process of machining is performed along the circumferential directions of the internal surfaces of bent pipes so as to always guarantee that the central line of the bent pipe passes through the circle center of an end of the bent pipe. The method disclosed by the invention is stable in cutting state, good in surface quality, and high in efficiency, and breaks through the limitation that special equipment is necessary to be used. By using the method disclosed by the invention, the interference problem existing in processing the internal surfaces of bent pipes is solved, and the cutting impact load is effectively reduced, therefore, the method is suitable to be used for machining bent pipe components with a large diameter ratio and a large angle of bending.