To evaluate the mechanical properties of ceramic composites affected by coating defects, the SiC coated plain woven C/SiC composite by precursor impregnation and pyrolysis (PIP) process was taken in consideration. Firstly, the observation and statistical analysis of initial defects on SiC coating were performed, then a non-stress oxidation test at 900℃ was conducted, and the damage status of carbon fiber around coating defects was obtained. According to the actual characteristics of the oxidation test, a microscale model of SiC matrix and carbon fiber containing typical coating defects was established. Based on the evolution of oxidation interface under diffusion-controlled reaction mechanism, a simulation on the development of fiber overall damage was carried out and stiffness reduction was calculated. The results show that oxy-gases entered from coating defects diffuse through inner pores of composites and react with carbon fiber, causing tensile modulus reduction in consistency with weight loss, both of which are capable of evaluation on the oxidation extent of composites. The overall damage topography of carbon fiber is determined by the types of coating defects. Under the same distribution scale, damaged zone induced by cracking defects contains larger range of stress concentration compared with that induced by spalling defects. Judgement on the properties of thermal structure at high temperature is supported with the comparison of oxidation damage induced by different coating defects.