Abstract: According to the solidified characteristic of peritectic steel for wide and thick slabs, mold fluxes with high basicity, high crystallization rate, low crystallization temperature and low heat flux density were developed and experimentally studied on 3^# Caster at BaoSteel. The trial results of 4 casting times showed that experimental strands produced with the developed mold fluxes had not surface longitudinal cracks, but the surface longitudinal crack ratio of strands produced with original mold fluxes was 5%. The developed mold fluxes can prevent surface longitudinal cracks in wide and thick slabs effectively.