Additive manufacturing combined with reactive sintering can be used to fabricate extremely lightweight silicon carbide ceramic mirrors. However， the mechanical properties of silicon carbide prepared by this method， such as flexural strength and elastic modulus， are low and insufficient. A novel method was proposed to improve the properties of silicon carbide by high temperature oxidation. First， silicon carbide prepared by additive manufacturing and reactive sintering was oxidized at 850 ℃. Thereafter， the effect of oxidation time on material composition and surface defect content was studied. The enhancement mechanism of oxidation time on the properties of silicon carbide was elucidated. The results demonstrate that when the oxidation time is 2 h， a dense silica film is in-situ grown on the surface of the material， which can render the surface defects of the material to self-heal and effectively reduce their contents. In addition， the final body obtained the best comprehensive properties by this method， and the flexural strength and elastic modulus are 263.9 MPa and 384.75 GPa， with increments of 10.7% and 14.4%， respectively. This method has the advantages of high efficiency， low cost， and easy operation. The study provides theoretical guidance for improving the performance of additive manufacturing silicon carbide ceramics.