Abstract: Thermal stress, DSC, FTIR, element analysis (EA), XRD, mechanical properties and density were used to analyze the thermal reaction characteristics of large tow polyacrylonitrile (PAN) precursors (48K) in combination with small tow PAN precursors (24K). The large tow carbon fibers were prepared by 50 min continuous pre-oxidation method, in which the evolution of structure and properties were studied. The results show that the thermal stress of large tow PAN precursors is 1.13-1.43 times of small tow PAN precursors, and the starting temperature is lower. The difference of thermal stress reaches maximum at 250℃ and the corresponding density of large tow fibers is 1.316 g/cm³. The crystal regions of PAN precursors transform into amorphous regions rapidly at the initial stage of reaction, and the grain size of the crystal regions increases first and then decreases. The monofilament tensile strength and modulus of large tow carbon fibers prepared by 50 min continuous pre-oxidation are 4240 MPa and 244 GPa, respectively, which are at the same level as those commercial foreign large tow carbon fibers.