Abstract: With the investigation of continuous slab casting factory, the slab broadening phenomenon of different degrees in the continuous casting process was found, and the broadening amount could not be corrected through the adjustment of process parameters. According to the correlation between the mould size and slab broadening, the mould setting formulas for correcting the broadening amount were obtained;combined with the width of slab measured at the exit of caster, the mould setting data for correcting the broadening amount could be obtained to control the width of the slab. A specific application is shown combined with the parameters of the secondary continuous slab caster in Ma'anshan Iron and Steel Co., Ltd.