A new process for separating propylene and propane in ethylene production has been put forward in which the extractive distillation with aqueous acetonitrile (ACN) as entrainer is employed. A PRO/II software process simulation has been made with the selection of an appropriate vapor-liquid equilibrium model. The new process, extractive distillation, requires a small number of trays, less energety consumption and small column diameter. In the design of the distillation column a new type of multi-overflow compound slant-hole tray was adopted which has been widely used in the recent years. It is believed that with the new process the industrial separation of propylene and propane is greatly improved.