Disclosed is a reforming system using an off gas as a cooling medium, which includes: a compressor configured to compress a feed gas; a cooling system a heat exchanger connected to the compressor and configured to cool the feed gas, the temperature of which has been raised in a compression process, by a cooling medium including cooling water; a reformer configured to generate a synthesis gas including hydrogen by reacting the feed gas, which passed through the heat exchanger, with water; a pressure swing adsorption (PSA) unit configured to separate hydrogen from the synthesis gas generated by the reformer and discharge the off gas; and an off gas line configured to feed the off gas discharged from the PSA unit to the heat exchanger such that the heat exchanger utilizes the off gas as the cooling medium.