Based on the calculation results of bloom thermal shrinkage, and combed with the roll gap deviation of withdraw machines measured by bloom thickness scale, the one gap on-line calibration method for withdrawal machines of bloom continuous caster was studied and developed, guaranteeing the accuracy of soft redcution amount of strand. The application of this method shows that the relative implementing deviation of soft reduction amount was controled no more than 4%, and the zero devation rate of bloom thickness reached 83.33%.