In order to study the quantitative relationship between the surface characteristics of carbon fiber and the interfacial shear strength of carbon/epoxy composite. The surface morphology, specific surface area and surface chemical characteristics of T800-grade carbon fibers were characterized by scanning electron microscopy, specific surface area analyzer and X-ray photoelectron spectroscopy. The interfacial shear strength (IFSS) of the composites was characterized by the micro-debond test. Based on the test results of specific surface area of carbon fibers, the real interface area was calculated and taken into account, while the influence of interfacial area and chemical characteristics on the IFSS was analyzed. The results show that there are obvious differences in the specific surface area of T800-grade carbon fibers with different surface conditions, and the difference in specific surface area of the carbon fibers treated by the two desizing methods is 25.4%. It shows a good linear correlation between the real interfacial shear strength (IFSS'), calculated based on the real interfacial area, and the surface oxygen-carbon ratio of carbon fiber, and the coefficient of determination reached 0.94. It is demonstrated that increasing the specific surface area and the surface oxygen-carbon ratio of carbon fiber are effective methods to improve the interfacial shear properties of composites. Moreover, it also provides a new way to quantitatively study the influence of physical and chemical characteristics of the carbon fiber’s surface on the interfacial properties of the composites.