Abstract: To address the poor surface quality and the challenge of achieving ultra-precision post-processing for knee implants fabricated by selective laser melting (SLM), a five-axis magnetorheological shear thickening polishing (MRSTP) method is proposed. A five-axis polishing system was designed and developed based on a programmable multi-axis controller (PMAC). A multi-pole coupled polishing tool was designed and optimized through simulations performed in Maxwell software. The magnetic field distribution characteristics and the feasibility of the device were verified using a gaussmeter. A 316L stainless steel knee implant produced by SLM was used as the workpiece to investigate the influence of different process parameters on material removal. Based on a first-order exponential decay kinetic model, the polishing rates under different process parameters were calculated and analyzed. Finally, the optimal combination of process parameters was selected for polishing experiments. The surface roughness Sa was reduced from an initial value of 705 nm to 45 nm. Surface scratches and SLM-induced pits were completely removed. This work provides a theoretical basis and technical support for the ultra-precision post-processing of additively manufactured medical implants.