The pyrolysis behavior of polycarbosilane(PCS) and chemical reaction mechanism during the pyrolysis process were studied by thermogravimetric-mass spectrometry(TG-MS) combined with X-ray diffraction and infrared spectroscopic analysis methods. The experimental results indicate that the main gas phase products generated during pyrolysis of PCS in nitrogen atmosphere include H2,-CH3 and CH4. The heating rate has a large effect on the pyrolysis process of PCS, the lower heating rate releases more small molecule gases and gets bigger rate of pyrolysis mass loss, demonstrating that the lower heating rate is beneficial to fully pyrolysis of PCS and obtain ceramics products with better microstructure.