Due to the excellent mechanical properties and heat resistance, poly(p-phenylene benzobisoxazole) (PBO) fiber is applied as one of most potential reinforcement in resin matrix composite. However, the poor adhesion with resin limits its application in advanced composite materials. In this paper, direct fluorination of PBO fiber was carried out for the first time, and the effect of surface moisture on the direct fluorination of PBO fiber was studied by controlling the moisture content of PBO fiber. The surface chemical structure and physical morphology of PBO fiber were investigated by X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FT-IR) and scanning electron microscope (SEM). It is shown that the surface polar groups of fluorinated PBO fiber increase obviously, and with the increase of surface moisture content, the ratio of polar groups (including COOH and C-F) decreases from 4.93% to 0.76%. Correspondingly, the interfacial bonding strength of fluorinated PBO fiber and epoxy resin decreases from 27.1 MPa to 21.4 MPa, but higher than 19.6 MPa of the original PBO fiber.