Carbon fiber reinforced polymer (CFRP) composites have attracted considerable attention because of their high specific mechanical properties. A promising application of CFRP is in marine and offshore area, where the satisfactory long-term performance of the composite is highly dependent on the interlaminar properties and matrix degradation due to moisture intake. This paper presents a systematic study of how moisture absorption and fiber surface treatment affect the interfacial properties of CFRP. A well-defined and characterised treating method combing silane coupling agent together with multiwalled carbon nanotubes (MWCNTs) is used to modify carbon fiber surfaces. Carbon/epoxy laminates were fabricated using vacuum assisted resin transfer moulding method and then placed in hygrothermal environment at 80 °C for 60 days. Mode I and Mode II interlaminar fracture toughness and interlaminar shear strength (ILSS) were evaluated through double cantilever beam (DCB) tests, end-notch flexure tests (ENF) and short beam shear (SBS) tests, respectively. The obtained results show that ILSS, mode I and mode II interlaminar fracture toughness, and glass transition temperature (Tg) of laminates decrease after enduring the hygrothermal aging. It is found that silane treatment can improve the interlaminar fracture toughness of laminates, and incorporation of MWCNTs can further enhance these interfacial properties. The average values of mode I and II fracture toughness and interlaminar shear strength of the laminates are 33%, 40.5% and 4.3% higher than the untreated samples after hygrothermal aging, respectively. However, surface treatment shows insignificant effect on Tg of composite laminates, probably because these properties were mainly dominated by epoxy matrix.