Ultrafast dynamics and its excitation-energy-density dependence of photoexcited carriers in amorphous Ge2Sb2Te5 film were studied at room temperature by femtosecond-time-resolved pump-probe reflectivity spectroscopy. It was found that the reflectivity reduced down to a minimum in 05 ps after pump excitation, and then started to increase sharply up to a new maximum value larger than the initial reflectivity in several picoseconds. Furthermore, the decreased and increased amplitudes of the reflectivity with respect to its initial value as well as its rising rate from the minimum to the maximum both increased with the increasing excitation energy density. The sharp rising process of the reflectivity was explained quantitatively based on the model of Auger recombination of high density plasma and Auger-recombination-induced heating of cyrstal lattice, which shows that the heating effect induced by Auger recombination in high density plasma should be the main mechanism of ultra-short laser pulse-induced phase change.