According to the heat transfer analysis of Airslip mold, an analytical model was presented to study the solidification shell thickness and the critical solidification shell thickness. The theoretical analysis and experimental results indicate that the product of solidification shell thickness and the casting rate is a constant, when the casting rate increases, the solidification shell thickness decreases and the billet surface is better, the variation of the casting rate brings the fluctuation of solidification shell thickness and therefore deteriorates the billet surface. It is also shown that the product of billet diameter and the casting rate is a constant, too. When the billet diameter increases, the casting rate should be reduced correspondingly.