The invention belongs to the field of lithium ion battery separators, and relates to a separator preparation method for a lithium ion battery with high safety and high energy density. The separator consists of special engineering plastics polyphenylene sulfide. The invention discloses the preparation method of the separator. The method adopts a thermally induced phase separation method of a double-component diluent, and includes melt blending the double-component diluent and the polyphenylene sulfide; quenching and pulverizing the mixture into powder; then pressing the powder into a separatorby a flat vulcanizer; and finally dring the separator after removing the diluent by using an extracting agent to obtain the polyphenylene sulfide separator. The preparation method for the separator issimple in process, low in production cost and easy for large-scale production. The polyphenylene sulfide separator has the characteristics of large porosity, high temperature resistance, high mechanical strength, good electrolyte wettability and the like, and is of great significance for developing the lithium ion battery with high safety and high energy density.