The invention discloses a method for splicing and processing an optical element by ion beams, which comprises the following steps of acquiring a removal function by experiments, acquiring a surface-shaped error function by a wave-front interferometer, dividing the processing area of an element, preparing data, confirming area frontiers, correcting the surface-shaped error, establishing a model, calculating out the residency time, processing the first shape correction according to the calculated-out residency time, identifying and error of the first shape correction processing, and repeating the processing step according to the condition of the error identification result until the requirements of the surface-shaped precision is satisfied. The processing method of the invention improves the processing capability of the prior optical processing system, solves the technical problem of processing large-caliber optical elements by the prior small optical processing system and can greatly lower the production cost and the processing cost of the optical processing system.