Smooth, uniform and controllable hydrothermal carbon coatings (HTCCs) had been successfully constructed on the carbon fibers through the efficient hydrothermal transformation approach from renewable carbohydrate. The thickness of HTCC could be effectively tailored from nano-scale to micro-scale by just adjusting the concentration of carbohydrate solution and the oxygen content in the HTCC could be significantly controlled through the high-temperature treatment. As for the growth model of the HTCC, FeCl3 played a critical role in improving the transformation from glucose to the dehydrated intermediate, mainly composed of 5-hydroxymethylfurfural (HMF), and promoting the conversion from HMF to carbonaceous structure, subsequently. The efficient and scalable preparation route not only offered promising prospects for radically substituting the traditional methods to prepare the carbon coating, but also provided a novel surface modification strategy of carbon fibers in the fields of carbon fiber reinforced composites and flexible textile electronic devices.