Carbon fiber-reinforced polymer (CFRP) material is increasingly being used for the repair of steel structures, mainly because of their superior mechanical properties. However, there is insufficient information on the environmental durability of CFRP-steel bond, especially in hygrothermal aging combined with sustained load. In this paper, 500 mm long І steel beam specimens reinforced with externally bonded wet-laid CFRP sheets are investigated. The specimens are subjected to a sustained four point flexural load of about 30% of initial ultimate load, placed into hygrothermal condition. The ultimate bear capacity and the failure characteristics of the CFRP and steel interface are studied after hydrothermal age and loading. The test results show that the coupled action of sustained load and hygrothermal aging reduces the bond strength of CFRP-steel, even for relatively short conditioning periods, and appears to significantly affect the failure characteristics of specimens.