The invention belongs to the field of wide and thick plate continuous casting machine soft-reduction process in steel production, and particularly relates to an on-line calibration method for the opening of a wide and thick plate continuous casting machine soft-reduction segment. The on-line calibration method is mainly characterized by including the steps: pressing both an inlet and an outlet of the segment to the mechanical lowest position; measuring the actual opening of the segment; comparing the opening; determining a calibration value; calibrating the opening in an on-line manner; and calibrating the opening again. The on-line calibration method has the advantages that by the aid of the on-line calibration method for the opening of the segment, the opening of the segment can be accurately calibrated, a preset soft-reduction scheme can be accurately implemented to obviously improve the low power quality of a casting blank, the rate of blank samples with deviation within 0.5mm from a set roll gap can reach 100%, the rate of blank samples without deviation from the set roll gap can reach 87.60%, center line shrinkage and middle cracks are eliminated, and center segregation and center porosity are decreased.