In order to shorten the processing cycle of the cycloid gear in the unknown modification mode,reduce the production cost and improve the processing efficiency,a high-order piecework fitting algorithm was proposed.The algorithm could be used to reconstruct the cycloid gear profile in the unknown modification mode by means of reverse engineering.In order to verify the rationality of the algorithm,tooth profile reconstruction for the profile points of RV320 cycloid gear tooth in the known modification mode was completed by using a high-order piecewise fitting method,then RV320 cycloid gear model was worked out by VERICUT simulation.The RV320 cycloid gear simulated model was compared with RV320 cycloid gear theory model,the largest processing error was about 0016 86 mm.After that,RV320 cycloid wheel was processed and its accuracy was detected.The detection results show that the cycloid wheel processed by the high-order piecewise fitting method meets the use requirements,so the rationality of the algorithm was further verified.Thus,the use of high-order piecewise fitting method can also be used to reconstruct the tooth profile of the cycloid gear in the unknown modification mode,and also provide a method for the high precision machining of the cycloid gear in the unknown modification mode.