Abstract: The microstructure of pre-hardened 718 plastic die steel bloom with a cross-section size of 1 360 mm×715 mm was studied using OM and SEM.The surface morphology of different microstructure were characterized by AFM.The polishing removal mechanism of the microstructure containing tempered bainite and pearlite was analyzed.The results show that the microstructure at the edge of the tested bloom was uniform tempered sorbite.Segregation banding was found in the area between the edge and the core,and the microstructure in this area was tempered bainite,pearlite,sorbite + granular carbide in segregation banding.The microstructure in core was pearlite.The tempered sorbite had excellent polishing performance due to fine microstructure and uniform carbide distribution.The polish performance of tempered bainite decreased because of containing large bainitic ferrite laths and uniformly distributed coarse carbides.Pearlite and abnormal microstructure in segregation banding had poor polishing performance,the morphology like orange peel could be observed.The larger the yield strength difference of the microstructure on the surface having different microstructure,the poorer the surface polishing performance.