In order to expand the industrial application of diamond coating tool and improve the adhesive strength of diamond coating tool. In this paper, the “Acid-Murakami-Acid” (AMA) three-step pretreatment process for cemented carbide with different WC grain size of 0.2, 0.4, 1.0μm was systematically studied. The surface morphology of the pretreated substrate, cobalt content and coating morphology were analyzed by scanning electron microscope and EDS spectrometer. The phase structure of the coating was analyzed and characterized by Raman spectroscopy and X-ray diffraction spectroscopy. The scour resistance of the diamond coating was tested by dynamic impact experiments. These results showed that the acid treatment played an important role in removing Co. The smaller the grain size of WC, the longer the acid treatment time was required. The etching capacity of WC by Murakami treatment increased at first and then decreased, and the maximum Co exposure was reached at 3min. Therefore, the best pretreatment process of WC-6%Co (0.2, 0.4, 1.0μm) substates were determined. After the three-step method, the substates obtained uniform and compact diamond coatings with grain orientation of (111), excellent scouring resistance and great adhesion strength.