In response to the technological constraints of traditional magnetorheological finishing machines， which typically feature an upper-mounted polishing wheel and encounter difficulties in processing optical components with significant steepness， this study introduces a compound linkage polishing technology. This innovative approach integrates a fixed mechanical axis with a virtual axis and employs an under-mounted fixed polishing wheel. Initially， a post-processing model for the compound linkage polishing of both the mechanical and virtual axes is established， informed by the structural characteristics of the under-mounted fixed polishing wheel. The model is articulated using the Denavit-Hartenberg （DH） method and is subsequently validated through geometric analysis. The accuracy of the established post-processing model is further corroborated by conducting spot-picking experiments on a virtual shaft concave spherical surface and uniform polishing experiments on a fused silica concave spherical surface， both with a diameter of 150 mm and a radius of curvature of 150 mm. Ultimately， a concave spherical surface crafted from molten fused silica， with a diameter of 170 mm， a radius of curvature of 158 mm， and a maximum normal angle of 32.54°， is polished and refined. Experimental results indicate that the PV value of the workpiece surface， with a trimming of 5 mm， converges to 0.04λ， while the RMS value converges to 0.005λ post-modification. These findings highlight that the proposed combined polishing model of the mechanical and virtual axes provides high precision and significantly enhances the processing capabilities for high-precision， high-gradient curved optical elements. This research contributes valuable insights into the application of magnetorheological polishing technology in the fabrication of complex curved optical elements with high steepness.