The invention discloses a method for reliquefying vapor of shipped liquid cargos. The method comprises the following steps: performing heat exchange in a cold box between low pressure vapor from a liquid cargo tank and high pressure vapor which is compressed and has heat exchange with sea water in a second step; performing two-stage compression on the low pressure vapor of which the temperature is increased through the heat exchange with the high pressure vapors, and performing heat exchange through sea water after each stage of the compression; raising the pressure of the low pressure vapor so that the lower pressure vapor becomes the high pressure vapor and comes into the cold box to perform heat exchange with the low pressure vapor from the liquid cargo tank so as to cool down the vapor; and performing heat exchange in the cold box between the high pressure vapor pre-cooled by the low pressure vapor and a condensing agent, and condensing and liquefying the high pressure vapor. The method has the advantages of low energy consumption, simple equipment and high recovery rate. A reliquefying system designed according to the method can reliquefy different types of liquid cargo vaporto satisfy the transportation of more petrochemical products so that ships equipped with the system are more competitive.