This work aims to investigate the characteristics of atmospheric pressure helium/oxygen mixed plasma jet with different oxygen content to analyze their effects on the surface physical and chemical properties of carbon fiber composites (CFRP) and study the mechanism of improving the surface wettability and bonding strength. The surface of CFRP was treated by helium/oxygen mixed plasma jets produced by the atmospheric pressure dielectric barrier discharge (DBD) to study the effect of different oxygen content on CFRP surface, thus determining the relatively better oxygen volume fraction. By means of contact angle measuring instrument, scanning electron microscope (SEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS), the surface wettability, micro-morphology, roughness and chemical composition before and after helium/oxygen mixed plasma jets treatment were measured and analyzed. Epoxy resin adhesive was used to bond CFRP and aluminum alloy surfaces before and after helium/oxygen mixed plasma jets treatment, and the bonding strength of different surfaces was tested. With the increase of oxygen volume fraction, the jet length and temperature decreased gradually; after oxygen was mixed, the CFRP surfaces with better wettability were obtained; when the oxygen volume fraction was 0.75%, the wettability was relatively optimum. Compared with pure helium plasma jet, the spreading rate of epoxy resin on the surface treated by helium/oxygen mixed plasma was higher, which indicated that the surface has better affinity for adhesive and its roughness also reduces. XPS results showed that the surface treated by helium/oxygen mixed plasma had higher oxygen- containing functional group content and higher surface energy, thus the surface wettability was better. When the oxygen volume fraction is 0.75%, the helium/oxygen mixed plasma jet can improve the surface wettability of CFRP, the surface contact angle decreases from 15.2° to 0°; the surface is smoother and the bonding strength increases by about 429.1% compared to the untreated surface.