When the operating vacuum is about 500 Pa, it is capable of conducting in the scratch film evaporator or falling film evaporator. However, if the distillation is operate under the high vacuum status ( for instance, the pressure ranges from 0.001 to 100Pa), problems are bound to cause. Therefore, it is necessary to choose the evaporator which the vapor pressure id no less than pressure difference caused by heating and cooling surface, and the evaporator quipped with outside condenser is excluded from the distillation process under the high vacuum range.