The SiC coating on carbon fiber was synthesized via an in-situ reaction method using silicon powder as silicon source, and the effect of reaction pressure on the synthesis, microstructure, and oxidation resistance of SiC coating was investigated. The results show that SiC coating synthesized at the atmospheric pressure is loose and porous; numerous SiC nanowires are formed on it. In comparison, the SiC coating synthesized at the low pressure is uniform and dense. The isothermal oxidation test shows that the SiC coating synthesized at the low pressure has better oxidation resistance than the one synthesized at the atmospheric pressure, because the dense and uniform SiC coating can prevent oxygen from contacting with carbon fiber more effectively. Based on the experiment results, the growth mechanism of SiC coating influenced by the reaction pressure was proposed. At the atmospheric pressure, the kinetic energies of deposition particles are too low to overcome the shadowing effect, causing the formation of porous SiC coating. By contrast, the deposition particles have higher kinetic energies and surface diffusion rate on the surface of carbon fiber at the low pressure, thereby forming a uniform and dense SiC coating.