Ge2Sb2Te5 (GST) alloy films on Si (100) substrate was prepared by a magnetron sputtering system at room temperature. The samples irradiated by femtosecond laser with different energy or annealed at 200℃ in an annealing furnace were studied by Raman spectra measurement, respectively. The dynamic transformation from amorphous state to crystalline state of GST films was analyzed by the changes of their Raman spectra. With the increasing of the laser intensity irradiated on the samples, regular shifts of their Raman peaks were found. Furthermore, the Raman spectra of the samples annealed at 200℃ are similar to that of irradiated by femtosecond laser with intensity of 24 mJ/cm2. The phase change caused by ultrafast laser irradiation is similar to the thermal treatment in this case.