The invention relates to a synthetic method for heptafluoropropyl trifluoromethyl ether. The synthetic method comprises the following steps: with trifluoromethyl hypofluoride and perfluoropropylene asraw materials, subjecting gaseous trifluoromethyl hypofluoride and liquid perfluoropropylene to generate a free-radical addition reaction at a low temperature so as to generate the heptafluoropropyltrifluoromethyl ether. The invention provides a chemical method to synthesize perfluoroalkyl ether. The method provided by the invention has the following advantages: the use of highly-active fluorinegas or an expensive catalyst is not needed; a previously-produced mixture of trifluoromethyl hypofluoride and fluorophosgene can be utilized for polar synthesis; production steps are simplified; meanwhile, the perfluoroalkyl ether has the advantages of high yield, simple synthetic method, and a certain industrial popularization value.