A mathematical model of the direct chill (DC) casting process for superlight Mg-Li alloy (LA141) slab has been developed using the finite differential method (FDM). Thermal boundary conditions including primary and secondary cooling conditions, have been selected based on knowledges of the physical process and the literatures. The variations of heat transfer coefficient between slab and dummy block interface resulting from the deformation of the slab have been considered. By calculating the temperature distribution, solid-liquid interface shape and position, the influence of casting parameters on DC casting process, such as the pour temperature, cooling water flow rate and casting speed have been analyzed. Moreover the profiles of the solidification fronts during the casting processes for the alloys of LA141 and AZ31 are compared.