In the hard turning process of AISI M2, due to its high hardness and high strength, the tool wear is serious. Tool wear seriously restricts the economic feasibility of hard turning and the machining quality of workpiece. Therefore, it is very necessary to take optimization measures of cutting parameters to reduce tool wear. In this research, 15 groups of hard turning experiments were carried out based on BBD. The relative wear of tool flank in hard turning per unit material volume was taken as the optimization objective, and the prediction model of relative wear of tool flank was established. The research results were as follows: (1) the wear forms of CBN tool were crater wear of tool rake, abrasive wear of tool flank and cutting edge wear; (2) the correlation coefficient R2 of the prediction model was 0.9922, and the adjusted correlation coefficient Radj2 was 0.9843; and (3) the contribution of feed rate to the relative wear of tool flank was 48.83%, and that of depth of cut was 22.05%. A = 81.17 m/min, B = 0.15 mm/rev and C = 0.2 mm were the optimal cutting parameters. This research is helpful to reduce the tool wear in hard turning and improve the economic feasibility of hard turning precision machining.