This invention discloses a preparation method for hexafluoropropylene oxide (HEP). The said method includes: a) providing solution for HFP in the reaction, and b) heating and charging oxygen, and preparing hexafluoropropylene oxide through molecular oxygen liquid-phase oxidation, wherein the referred reaction medium is selected from alpha-perfluoro-n-butyl-oxa-perfluorocyclopentane, alpha-perfluoro-n-propyl-oxa-perfluorocyclohexane or fluorocarbon cyclic ether mixture. The conversion rate of products can be improved and the reaction time can be shortened by this method.