The work aims to obtain smooth surface and improve the concentration of the reflected light path of aluminum alloy (Al 6061) cone mirror, adopt shear thickening polishing (STP) to remove the surface defects including cutting marks, pits, and humps after diamond cutting and eliminate the light path scattering caused by these defects. The influence of four key parameters in the STP process including polishing speed, composition of CeO2 abrasive (large particles with average size of 1.6 μm)/(small particles with average size of 200 nm), abrasive concentration and pH of shear thickening polishing slurry (STP slurry) on the surface roughness (Ra/Rz) was analyzed by Taguchi method. At the same time, the experiment to observe the concentration of reflected light was carried out and the effect of surface morphology on the concentration of reflected light was discussed. According to the analysis of S/N ratio, under the experimental conditions, the pH value of STP slurry had the greatest influence on the surface roughness after polishing, followed by polishing speed and CeO2 abrasive concentration. While, CeO2 abrasive composition had the least influence. The optimized polishing parameters with 7∶3 of large/small abrasives, 4.5wt% abrasive concentration, 1.38 m/s polishing speed and pH 5 of STP slurry were determined. After polishing for 20 min, the surface roughness (Ra) was reduced from 64.9 nm to 8.1 nm, and the surface defects were removed. The defects on the surface of aluminum alloy cone mirror after diamond cutting can be effectively removed after STP process. The hard prase (Mg2Si) of aluminum alloy material has an important influence on the surface morphology after polishing. Under the optimized conditions of STP, through the balance of chemical reaction and mechanical removal of abrasive, matrix material and hard phase of the workpiece can be removed at the same time, and a smooth surface without pits and humps can be obtained.