The invention belongs to the technical field of numerical control processing, and discloses a part processing size predicting method for a numerical control machine tool. The predicting method comprises the steps that software communicates with a numerical control system and collects big data of the machine tool during processing operation; the data of the big data of the machine tool and millingprocessing conditions are divided into two parts; graphic feature fitting is carried out on the first part of the data to acquire graphic size data; the second part of the data are calibrated with a size error measured by a measuring instrument used after processing, and a prediction model is established; and the reality is combined, and the distribution weights of a size error fit by the first part of the data and a size error evaluated by the prediction model established by the second part of the data are added to acquire a final online size error prediction model. According to the invention, used measuring instruments can be reduced; labor cost and equipment maintenance cost are saved; the size error result of each processed part can be conveniently and quickly given online, and the precision is high; and the production efficiency is improved.