Abstract: Carbon fibers are excellent materials with light weight and high mechanical strength, and have already been widely used in the automotive industries, sports apparatus and the field of aerospace. Polyacrylonitrile (PAN) precursor fiber is one of the most popular raw materials for the production of high performance carbon fibers. Generally, the manufacturing process of PAN based carbon fibers consists of three main steps: peroxidation, carbonization and graphitization. Among these, peroxidation is the most complex and time-consuming step, which has a great influence on the final properties of carbon fibers. In this work, the thermal stabilization processes of PAN precursor fiber in both nitrogen and air atmosphere were studied using pyrolysis single photoionization time-of-flight mass spectrometry (Py-SPI-TOF MS) and thermogravimetry-mass spectrometry (TG-MS). TG-MS was used to identify different decomposition stages of PAN precursor fiber and to evaluate the evolved gases simultaneously. As to the products with relatively higher molecular weight (ionization energy 10.6 eV), Py-SPI-MS was applied to analysis. Thanks to the “soft” near-threshold photoionization character of SPI-TOF MS, few or even no fragments of molecular ions can be formed in the ionization process. This makes the identification and interpretation of complex decomposition products in real time possible. The mass spectra of decomposition products at isothermal temperatures and temperature-evolved profiles of selected species during the thermal stabilization processes were measured. The experimental results indicate that cyclization structures were formed without loss of any material. As to the thermal decomposition of PAN in nitrogen atmosphere, two evident thermal decomposition stages were observed as the heating temperature increased. The first stage can be attributed to the acrylonitrile (AN) chain scissions and liberation of nitrogen-containing gases, during which HCN, NH₃, AN monomers, dimers and trimers were generated. The decomposition of the unstable cyclization structures took place in the second stage, mainly producing cyclization compounds and light olefins. However, in air atmosphere, oxygen accelerated the formation and stabilization of the cyclization structures. Such a process suppressed further decomposition of the cyclization structures to a great extent. Moreover, in order to study the effects of thermal stabilization temperature, thermal decomposition processes of PAN precursor fibers under different heating rates were performed. The results indicated that the formation of cyclization structures prefers lower heating rates. These findings obtained with Py-SPI-TOF MS in real time prove that both oxygen and temperature condition play important roles in thermal stabilization processes of PAN fibers. Nevertheless, the application of Py-SPI-TOF MS is still limited since it cannot be used to detect compounds with ionization energy over 10.6 eV. Therefore, future works on the study of thermal stabilization of PAN fibers can be made with synchrotron radiation SPI-TOF MS, ionization source of which has a broad photon-energy range.