Abstract: Samples at different positions of prehardened large section 718 plastic mold steel were heat treated, and then mechanically polished and manually polished. The surface roughness was tested. The effect of microstructure on the surface polishing performance was studied. The results show that after heat treatment, the sample with the original microstructure of non-uniform granular bainite had the worst uniformity of microstructure and hardness distribution, leading to the largest surface roughness and the worst polishing performance after polishing. The sample with the original microstructure of uniform granular bainite after heat treatment had the best uniformity of microstructure, and the hardness was relatively large and distributed uniformly; therefore the surface roughness after polishing was the smallest. Moreover, no orange lines or pitting defects appeared on the surface after engineering polishing (professional manual polishing for engineering applications), and the limit of surface roughness reached 0.019 μm; the surface quality met the application requirements of high-end plastic mold steel. The uniform distribution of carbides in the sample with the original microstructure of tempered martensite after heat treatment made the surface polishing performance slightly better than that of the lower bainite sample.