Based on ASPEN PLUS, a new air separationtechnical process was designed. Three kinds of air separation products, industrial nitrogen, high purity nitrogen and liquid oxygen were prepared by policy-quasi-batch distillationgas and gas return expansion. With appropriate physical property method, this separation unit was designed and simulated with ASPEN PLUS when the equipment heights is restricted. In order to adapt to air separation in situ preparation, industrial nitrogen was used for preparing high purity nitrogen. And temperature, pressure and composition distribution of the distillation column were discussed. The results show when situ preparation air separation equipment heights is restricted, the cryogenic distillation of air is achieved by changing the arrangement of distillation columns and regulate reflux ratio etc..