In the technique of ion beam figuring, it often needs to extend the original surface error data to improve the dwell time solving precision on the edge. The virtual extended surface should be smoothly stitched with the original surface error data and should have controllable uncertain ability. Non-uniform rational B-spline (NURBS) surface is usually used in the mechanism fabrication field to model three-dimensional (3D) objects. Using the non-uniform rational B-spline surface and combining with the Zernike polynomial fitting method, a typical tested original free-form surface error data set is extended. The power spectral density curves of typical original surface error and the extended surface error show that the surface error is improved by more than 70% when the error spatial frequency is greater than 0.05 mm-1. This extended surface is also used in the dwell time algorithm, and the forecasted machining precision is improved from 1.18 nm to 0.19 nm in root mean square (RMS) value. The results show that smooth virtual extended surface can be got and the computing precision of the dwell time algorithm can be improved by using non-uniform rational B-spline surface to extend the surface error of the optical components.