The XRD, SEM, isothermal oxidation-weight loss and non-isothermal TG-DTG were used to study the properties of SiC coatings/ 3-D braided carbon fiber(composite fiber) prepared by in-situ reaction method. The results show that coatings is coated symmetrically and combine well with every fiber of 3-D braids in proper reaction conditions, the oxidation resistance of composite fiber increase with the increase of SiC coatings thickness. The oxidation resistant properties and mechanism of composite fiber were studied also.