Surface　oxidation　of　SnSe　sharply　reduces　its　thermoelectric　properties　though　the　bulk　single-crystalline　materials　of　SnSe　claim　the　record　high　zT　values.　Investigation　on　the　oxidation　behaviors　of　SnSe　together　with　the　subsequent　phase　transition　and　element　migration　is　fundamentally　important　to　maintaining　the　ultrahigh　zT　values,　with　a　potential　for　further　improvement.　In　this　work,　we　disclose　the　dynamic　epitaxial　crystallization　of　SnSe2　on　the　amorphous　surface　of　partially　oxidized　SnSe　crystals　and　the　corresponding　atomistic　mechanisms　via　transmission　electron　microscopy　(TEM).　It　is　revealed　that　the　thermally　annealed　amorphous　surface　crystallized　to　SnO2　and　SnSe2　in　the　outermost　and　secondary　layers,　respectively,　forming　distinctive　SnSe/SnSe2/SnO2　multilayer　heterostructures　with　specific　orientation　relationships　between　the　two　selenides.　By　means　of　in　situ　scanning　TEM　(STEM),　the　dynamic　epitaxial　crystallization　process　of　SnSe2　was　revealed　when　the　oxidized　SnSe　surface　was　subjected　to　electron　beam　irradiation.　Through　the　atomic-scale　characterization　and　modeling　analysis,　we　find　that　the　exposed　dangling　Se　diatoms　on　the　SnSe　surface　serve　as　nucleation　sites　for　lateral　epitaxial　crystallization　of　SnSe2.　The　same　valence　and　similar　coordination　configuration　of　Se　atoms　in　these　two　phases　are　supposed　to　facilitate　the　sharing　of　Se　atoms,　with　lattice　distortions　in　the　SnSe2/SnSe　interface.　These　findings　are　valuable　for　understanding　the　surface　oxidation　behavior　of　SnSe　and　revealing　the　interface　structures　of　SnSe2/SnSe　heterojunctions　and　also　offering　new　routes　for　SnSe-related　multilayer　or　heterostructure　system　design.