The　integrity　of　the　hydrogenated　amorphous　silicon/crystalline　silicon　(a-Si:H/c-Si)　interface　is　essential　for　the　enhanced　performance　of　a-Si:H/c-Si　heterojunction-based　devices.　However,　during　annealing　processes　aimed　at　passivating　silicon　dangling　bonds,　unexpected　Si　epitaxy　and　nanotwin　formation　tend　to　emerge,　even　under　low-temperature　conditions.　Therefore,　understanding　the　influence　of　such　annealing　on　the　a-Si:H/c-Si　interfacial　structure　is　therefore　pivotal　for　device　optimization.　In　this　study,　the　atomic-scale　structural　transformation　of　the　a-Si:H/c-Si　heterointerface　subjected　to　low-temperature　annealing　is　delved　into.　The　dynamic　evolution　of　this　interface　is　captured　by　employing　in　situ　spherical　aberration　(CS)-corrected　transmission　electron　microscopy　(TEM),　molecular　dynamics　(MD)　simulations,　and　density　functional　theory　(DFT)　calculations.　The　TEM　observations　indicated　that　Si　epitaxy　initiated　before　Si　nanotwin　formation,　and　these　nanotwins　are　inclined　to　revert　to　epitaxial　structures　upon　sustained　annealing.　Through　MD　and　DFT　insights,　the　thermodynamic　and　kinetic　intricacies　driving　the　concerted　tri-layer　atomic　shift　characterizing　the　Si　nanotwin-to-epitaxy　transition　are　decoded.　The　findings　shed　light　on　the　thermal　behavior　of　a-Si:H/c-Si　interfaces,　offering　new　perspectives　on　the　thermal　management　in　silicon　heterojunction　devices.