Phase-change　materials　(PCMs)　have　been　widely　used　in　optical　and　electronic　applications　for　their　extraordinary　properties.　However,　the　procedure　of　fast　transition　about　PCMs　is　still　not　clear　for　the　time　limit.　Pulse　laser,　especially　ultra-short　pulse　laser,　is　an　excellent　tool　to　study　the　crystallization　characteristics　of　PCMs　for　its　high　heating　rate.　In　this　paper,　picosecond　pulsed　laser　was　employed　to　study　the　crystallization　of　amorphous　Ge2Sb2Te5　(a-GST)　thin　films　via　the　evolution　of　morphology　and　microstructure.　The　threshold　for　inducing　crystallization　was　7.9　mJ/cm(2)　and　amorphization　occured　at　17.2　mJ/cm(2).　With　the　aid　of　high　resolution　transmission　electron　microscopy　(HRTEM),　the　growth　of　grains　were　observed　to　own　a　preferable　growth　direction　of　[100].　The　grain　size　and　laser　fluence　showed　an　exponential　relationship　under　the　ps　laser　pulse,　and　the　maximal　grain　reached　to　similar　to　5　mu　m　when　approaching　the　melting　point.　The　morphology　and　grain　distribution　of　GST　film　under　18.8　mJ/cm(2)　indicated　a　solid-state　phase　transition.　Besides,　the　matrix　of　a-GST　film　changed　into　crystalline　state　even　though　the　surface　melted　into　amorphous　state　when　irradiated　by　high　laser　pulse.　Finally,　the　reason　for　fast　transition　of　a-GST　film　under　ultra-short　laser　pulse　was　discussed　in　terms　of　heating　rate　and　temperature.　The　present　study　is　fundamental　for　the　understanding　of　the　fast　phase　transition　of　PCMs.