An original mathematical model was developed to study the behavior of bubbles and influence factors in continuous casting mold. The interactions between bubbles were considered in the model,including bounce off,coalescence and breakup. The effects of casting speed and nozzle immersion depth on bubbles behavior were investigated, including region of bubble dispersion, gas volume fraction and bubble mean diameter. The results showed that the initial bubbles can merge into a large bubble in the nozzle. With increasing the nozzle immersion depth, the region of bubble dispersion and gas volume fraction increase, but the average diameter of bubbles decreases under the same casting speed. With increasing casting speed,the law is same under the same nozzle immersion depth. Casting speed has greater influence than nozzle immersion depth on bubble diameter.