In order to solve the problem of the high surface longitudinal crack ratio of heavy peritectic steel slabs produced by the No.3 continuous caster at Baosteel,the physical properties of the original mold flux and the optimized mold flux were compared in a comprehensive way by using analytical measures,such as a slag film heat-flow simulator,a thermo-wire molten flux crystallization tester and an X-ray diffractometer in the laboratory.The results reveal that one of the major reasons for the cracks is the poor heat transfer ability of the original mold flux.However,the optimized mold flux with a high basicity features a high crystallizing rate,low crystallization temperature and low heat-flow density.Therefore,the optimized mold flux is more suitable for casting peritectic steel by the heavy slab continuous caster.The test results show that the slabs produced by using the optimized mold flux had no surface longitudinal crack in four test casts,while the surface longitudinal crack ratio of the slabs produced by using the original mold flux was 5%.The optimized mold flux can effectively prevent slab surface longitudinal cracks from occurring. In order to solve the problem of the high surface longitudinal crack ratio of heavy peritectic steel slabs produced by the No. 3 continuous caster at Baosteel, the physical properties of the original mold flux and the optimized mold flux were compared in a comprehensive way by using analytical measures, such as a slag film heat-flow simulator, a thermo-wire molten flux crystallization tester and an X-ray diffractometer in the laboratory. The results reveal that one of the major reasons for the cracks is the poor heat transfer ability of the original mold flux.However, the optimized mold flux with a high basicity features a high crystallizing rate, low crystallization temperature and low heat-flow density.Therefore, the optimized mold flux is more suitable for casting peritectic steel by the heavy slab continuous caster The test results show that the slabs produced by using the optimized mold flux had no surface longitudinal crack in four test casts, while the surface longitudinal crack ratio of the slabs produced by using the original mold flux was 5%. The optimized mold flux can effectively prevent slab surface longitudinal cracks from occurring.