The Lutetium-Yttrium Oxyorthosilicate (LYSO) is one of the most widely used scintillation crystal in the high-performance Positron Emission Tomography (PET) systems. The quality of the surface finish of the LYSO has an important impact on the light output, the decoding performance, the energy resolution and timing resolution of the PET detectors and systems. In this paper, we present an ultra-precise method for processing the surface of LYSO crystals. The hardness and elastic modulus of the crystals were initially measured using Nano indentation technology. The scintillators were fixed onto the plate in sparse, serried and continuous arrangements and polished using an alumina (Al2O3) and cerium oxide (CeO2) polishing solution with particles of varying size. We used a magnetorheological-polishing technique to polish the LYSO crystals. The polishing solution here included hydroxyl iron powder and hard abrasives. The hardness and elastic modulus of the crystals in question was, respectively, 11.18 +/- 0.50 and 155.78 +/- 4 gigapascals (GPa). A 3D optical surface profiler (3D-OPS) and an atomic force microscope (AFM) were used to evaluate the quality of the polished surfaces. The average roughness of Ra 0.55 nm measured by 3D-OPS was achieved using a precise plate grinding and polishing technique. The magnetorheological-polishing method also obtained an excellent roughness of Ra 0.75 nm (3D-OPS). Our report of the use of these processing technologies can serve as a foundation for further in-depth research regarding the optimal techniques for scintillator surface processing.