Both deformation temperature and strain rate are important factors for the damage behavior of Csf/Mg composites in the liquid-solid state. In present paper, Zener-Hollomon parameter, which incorporates a combined effect of temperature and strain rate, was introduced to Cockroft–Latham fracture criterion as a correlation coefficient. The correlation coefficient, which equals facture strain under uniaxial tensile stress state, was determined by a series of tensile tests of Csf/Mg composites in the high temperature solid state and semi-solid state, and fitted into a nonliner function of lnZ, thereby obtaining a modified Cockroft–Latham fracture criterion. Furthermore, the modified Cockroft–Latham fracture criterion was applied to the simulation of liquid-solid extrusion, and the predicted results show a good agreement with experimental observations in both the direction and length of surface cracks.