Abstract: The liquid steel will be leaked from the slit between the bottom of solidification mold and mother sheet during inversion casting process. By substitution of the actual liquid steel, the Pb-Sn alloy and the numerical simulation can be used to study the fluid flow in the inversion solidification mold, the mother sheet critical speed is determined for preventing the leakage from the slit of bottom. When the mother sheet speed is more than 0.06 m/s, Pb-Sn liquid alloy will not leak from the slit of the mold. Otherwise, the Pb-Sn liquid alloy can leak from the slit of the mold.