The invention provides a width-adjusting control method for the taper of a crystallize for adjusting the widths of a first narrow surface copper plate and a second narrow surface copper plate on an inner arc copper plate of the crystallizer. A width adjusting step comprises the steps: a taper adjusting step of adjusting the widths at the tail ends of the first narrow surface copper plate and the second narrow surface copper plate in a gradually decreasing manner; and a locating step of separately leaning and limiting the outer side walls of the first narrow surface copper plate and the secondnarrow surface copper plate. By adjusting the last half turn in the width adjusting step of the crystallizer to the taper structure which is gradually decreased, leaning and limiting the first narrowsurface copper plate and the second narrow surface copper plate and carrying out follow-up ingot casting work, the condition of longitudinal corner cracks of the formed ingot is reduced, the problem of breakout by leakage is solved, continuous casting production is smoother, and meanwhile, the workload of cleaning the longitudinal corner cracks by hand cleaning staff is reduced, and the operatingstrength of staff is reduced.