The study is aimed at the inorganic process of the cured PCS fiber, the preparation of SiC fiber. Much work is done on elemental fields of the affections of the amount of the cured PCS fiber and the protective gas flow etc on the pyrolysis. We find through the experience that it is the offgas evacuated in the process that plays an important role by accelerating pyrolysis and increasing the weight loss. The explanation is that the living radicals in the offgas prevent the split chain radicals from recombination and the chains turned to be shorter and more degradable.