Argon bubbles are usually injected into steel continuous casting mold to prevent the clogging of submerged entry nozzle (SEN), but some bubbles may be entrapped to form defects in the final slab. In order to provide a reference for improving the quality of steel, a mathematical model based on the Eulerian-Lagrangian approach with advanced bubble break-up and coalescence models was established to study the effect of operation conditions on bubble distribution in a steel continuous casting mold. A bubble break-up model based on a daughter bubble fraction, which is suitable for the continuous casting system, was considered. The mathematic model was validated by comparing of the size and number of captured bubbles with the plant measurements of previous work. The result shows that argon gas injection has obvious effect on the flow pattern in the upper recirculation zone of the mold. In the upper recirculation zone, the bubbles mean diameter decreases and the bubble number increases with increasing casting speed, and both of the bubble size and number increase with the increase of gas flow rate. From the result, it can be found that the number and diameter of bubbles arriving at the advancing solidified shell region increase with increasing casting speed. In addition, the increase of gas flow rate causes more bubbles arriving at the advancing solidified shell region, but has little effect on the size of bubbles.