Magnetorheological jet polishing (MJP) plays an important role in polishing complex cavities and special optical elements with high precision. However, the roughness distribution function that describes the variation with polishing time of the roughness value of every area in the polishing area has not been studied deeply. In this paper, the influence of the roughness distribution on the removal function of MJP in optics (with a roughness of less than 10 nm) and its evolution model in the spatial and time domains are studied. With the increase of polishing time, the surface roughness of the central area linearly increases, forming surface defects, such as pits. The roughness of the polishing area exhibits a limited growth trend. Verification experiments are carried out on BK7 glass. The results of the roughness distribution on the removal function prove the correctness of the model. The model laid a foundation; therefore, it has important guidance and reference value for the application to the whole aperture polishing.