In　this　paper,　crystallization　characteristics　of　amorphous　Ge2Sb2Te5　(a-GST)　films　induced　by　a　Gaussian　picosecond　laser　with　different　laser　fluence　were　carried　out　using　transmission　electron　microscopy　(TEM),　Raman　spectra　and　ab　initio　molecular　dynamics　(AIMD)　simulations.　TEM　observations　presented　a　solid-state　phase　transition　with　nanocrystalline　microstructure　at　lower　fluence,　while　a　liquid-solid　phase　transition　accompanied　by　an　ingot-like　crystalline　microstructure　was　found　at　higher　fluence,　including　a　central　coarse　and　an　outer　fine　equiaxed　regions　as　well　as　a　columnar　crystal　region　between　them.　In　spite　of　the　remarkable　difference　in　microstructure,　the　composition　of　different　regions　kept　constant　from　the　Energy　Dispersive　Spectrometer　(EDS)　measurements.　Different　phase　change　behavior　and　laser　fluence　affected　the　shift　of　Raman　peaks　due　to　the　phase　change　stress　and　thermal　stress.　A　laser　irradiation　coupled　AIMD　simulation　was　then　developed　to　study　the　preferential　nucleation　possibility.　AIMD　simulation　results　indicated　that　a　thermal　process　in　the　central　coarse　equiaxed　regions,　with　a　higher　cooling　rate　(10　k/ps)　generated　by　the　Gaussian　picosecond　laser　irradiation,　reduced　the　nucleation　rate　remarkably,　causing　crystal　nucleus　have　enough　space　to　grow　up　even　to　the　micrometer　scale.　This　is　different　from　the　morphology　induced　by　nano-　or　femtosecond　pulsed　laser.