The microstructural evolution, elemental composition and gas releasing character of preoxided polyacrylonitrile (PAN) fibers during continuous carbonization were investigated by Raman spectroscopy, XRD, XPS and TG-MS. Results indicate that the sp C-C bond length, crystal size and graphitization degree increase. The increase of orientation degree and the decrease of micropore volume are the fundamental reasons for the improvement of density and tensile strength of carbon fibers. Surface and global carbon contents of pre-oxidized PAN fibers, and both low temperature and high temperature carbonized fibers indicate that the loss of non-carbon atoms during carbonization propagates from the exterior to the interior of the fibers. TG-MS indicates that gases are released mainly at low temperature in carbonization. The cross-linking of trapezoid giant molecular chains to release nitrogen takes place at a high temperature during carbonization.