The mechanism of broadening of slab in continuous casting was studied by numerical simulations and experimental measurements in factories. The mechanism is derived by gradual exclusion of various factors related to the broadening of slab. It is concluded that the slab exposes to no constraint at the direction of narrow face. Because of the static pressure of molten steel, the slab deforms creepily in the direction that consequently results in the broadening of slab. The broadening of slab increases with casting speed and static pressure of molten steel. The decrease of secondary cooling intensity and strength of steel at high temperature also contribute to the broadening of slab. The micro-alloying plays an important role in improving the strength of steel and in reducing the broadening of slab. broadening of slab, continuous casting, secondary cooling zone, thermo-mechanical, viscoelastic-plasticCitation: Nomenclature p: static pressure of molten steel, Pa l: width of slab, m S: thickness of slab shell, 10 2 m q: load force, N : a unitless coefficient D: thickness of slab shell on narrow side, m T So : solidifying temperature of molten steel, K b: roller spacing, m  m : a unitless coefficient W e : deflection, unitless W qe : elastic deflection, unitless : amount of side bulging, 10 3 m T 0 : initial temperature, K  xe : elastic deformation in x direction, unitless  ye : elastic deformation in x direction, unitless  xye : shear deformation in xy plane, unitless  m : convergence coefficient, unitless T(z): function of temperature force field in z dirrection   shear deformation, unitless x, y, z: coordinate dimensions, m T 0 : initial temperature, K : elastic deformation in x direction, unitless T m : average temperature of slab shell, K D s : thickness of slab, m  ye : elastic deformation in x direction, unitless  xye : shear deformation in xy plane, unitless W Te : creeping deflection, unitless t: time, s R i : radius of the ith roller, m S i : thickness of slab shell at the ith roller, 10 2 m  i : deformation of slab at the ith roller, unitless Fu J X, et al.