Aiming at the border effect caused by ion beam polishing optical components, a suppression method based on polynomial fitting continuation is proposed. The initial surface shape is extended by the polynomial extension method, and the obtained extended surface shape is simulated. The root mean square value of the shape after the simulation is 5.20 nm. The K9 optical element with a diameter of 100 mm is processed by ion beam polishing technology, and the root mean square value of the surface shape after processing is reduced from 19.26 nm to 12.23 nm. Then, 8% of the processed surface shape is selected as the boundary surface shape, and the root mean square value of the boundary surface shape is reduced from 137.23 nm to 56.72 nm. The feasibility of this method is verified through simulation and experiment. This research can provide a new method for optical processing