Abstract: To improve the dimensional accuracy of products, a detailed analysis was carried out on the deformation laws of three round pass profiles in the high-speed wire rod reducing and sizing mill unit. The deformation characteristics and metal flow behavior of round rolled pieces in the sizing mill unit were discussed from two aspects: the distribution of elongation coefficient and the structural characteristics of pass profile. It is pointed out that the metal flow in the round-round-round pass profiles is different from the "reduction-spreading" mode in the oval-round pass profiles. It is a full-compression deformation along the circumferential direction of the pass profile. A detailed analysis was conducted on the influence of the rolled pieces from the last stand of the reducing mill on the final product quality after being rounded by two sizing mill stands. The causes of the unique final product defects in the reducing and sizing mill unit were analyzed, and it was pointed out that the key to the process control of the reducing and sizing mill unit lies in the precise control of the round rolled pieces in the last stand of the reducing mill, and the defects generated here cannot be eliminated by the sizing mill.