Abstract: Argon gas injection into a slab continuous casting mold through the submerged nozzle changes the mold flow region. Surface level fluctuation in the mold would lead to slag entrapment in billets. The influence of argon injection on slag inclusion quantities and types in billets was investigated by means of SEM and bulk sample electrolysis. The reasonable technological parameters, such as the quantity and distribution of argon injection, were analyzed and provided for avoiding slag entrapment in billets. The result showed that the mass ratio of slag entrapment to the inclusions in the mold was beyond 50 % and the optimized argon gas injection was significant to reduce slag entrapment.