Abstract: The relation between critical pulling velocity and its effect factor is made by heat transport theoretical analyzing of continuous unidirectional solidification. When the distance of spraying position of forced cooling ingot and the liquid-solid interface is shorter than 10 mm.(L ≤ 10 mm),the calculated result of Al and Al-1% Cu alloy continuous unidirectional solidification is approximately identical with experimental result. All of calcu-lated and experimental parameters are quiet matching, and the continuous unidirectional solidification can be stably performed. The way of raising pulling veloc-ity is also mentioned.