A previous numerical model has been expanded to predict the impurity concentration profiles during purification of bibenzyl by zone refining after several zone passes. It proves that the model is efficient through comparison of theoretical and experimental impurity concentration profiles within 10 passes. For all impurities in bibenzyl, 30 zone passes are sufficient for zone refining by theoretical computation under the experimental conditions. Through optimization of zone length for two typical impurities in theory, it shows that refining efficiency with varied zone length, adopting a longer zone length for the early zone passes, and a shorter zone length for the later passes, is higher than that with constant zone lengths in all passes. It provides a guide for purification of bibenzyl during processes of zone refining, theoretically.