Three-dimensional (3D) C/SiC composites with a CVD SiC coatings and a PyC interphase were prepared by low-pressure chemical vapor deposition method. The interphase was annealed and changed in thickness. Dualism, certainty and randomicity, in the evolution of C/SiC composites under complicated environments were investigated. The results show that residual strength and its fluctuation are important when evaluating environmental adaptability and reliability. The interphase and coatings are the key microstructural units most sensitive to oxidizing environments. The three parameters to be considered, in order of importance, are temperature, atmosphere and stress sensitivity. The most significant atmosphere is oxygen, followed by water and salt, while the most important stress parameters are fatigue/creep interaction, creep and fatigue. Stress promotes degradation of the composites due to opening of cracks in the coatings. An oxide film on the coatings is best to seal these cracks, and water can accelerate this sealing. However, a fatigue/creep interaction stress causes sealing failure, and then the environment including oxygen, water, fatigue and creep is the worse one of all. For the composites to be self-adaptable, the PyC thickness should be optimum for increasing annealing effect, and the coatings should be oxidized at a moderate rate for increasing resistance to oxidation. The temperature and the partial pressure of oxidizing gases control the moderate rate.