The present invention relates to a process for preparing hexafluoropropylene(CF3CF═CF2, HFP) from the pyrolysis of trifluoromethane(CHF3, R23) and tetrafluoroethylene(C2F4, TFE) and more particularly, to the process for preparing hexafluoropropylene from the pyrolysis of an admixture of R23 and TFE mixed in an appropriate molar ratio at below 900 which is lower than the conventional reaction temperature and longer residence time, after investigating the pyrolysis reaction of R23 and TFE by the computer simulation. The process for preparing HFP is performed by carefully controlling reaction temperature with heat balance resulted from an endothermic pyrolysis of R23 and an exothermic dimerization of TFE to prevent from carbon formation, recycling unreacted R23 and TFE in the product separated and purified from distillation column, adding fresh R23 additionally to keep an appropriate molar ratio of R23 and TFE, to improve a total yield of HFP and to minimize heat supply from outside.