Tungsten　subcarbide　(W2C)　is　widely　applied　to　industrial　catalysts,　military　industries,　and　aerospace　facilities　because　it　possesses　excellent　high-temperature　performance　and　superior　mechanical　properties.　However,　contradictory　data　on　the　crystal　structure　of　W2C　including　its　disordered　and　different　ordered　phases　have　been　often　reported　in　the　literature,　and　atomic-scale　understanding　of　W2C　polymorphic　structures　has　not　yet　reached　a　consensus.　Based　on　the　L′3-type　lattice,　we　have　performed　first-principles　calculations　to　study　the　stability　of　L′3-WC,　the　interaction　of　dilute　carbon　vacancies　in　L′3-WC,　the　stable　ordered　structures　of　W2C　up　to　a　unit　cell　containing　ten　formula　units,　and　the　phase　transition　among　five　stable　ordered　W2C　structures.　Our　results　indicate　that　carbon　vacancies　in　L′3-WC　can　exhibit　an　attractive　interaction,　which　provides　an　essential　driving　force　to　stabilize　the　ordered　structures　of　W2C.　The　high-temperature　disordered　β-W2C　with　L′3-type　lattice　is　more　likely　to　be　stabilized　by　configuration　entropy.　The　stable　ordered　W2C　structures　are　all　extended　in　the　ab　plane　of　the　L′3-type　lattice　rather　than　along　the　c　axis　and　possess　some　specific　distribution　patterns　of　aggregate　carbon　vacancies.　New　ordered　W2C　structures　with　formation　energies　lower　than　those　of　β″-W2C　and　ϵ-W2C　are　found　to　be　mechanically　and　dynamically　stable.　Carbon　atom　migration　between　the　interlayers　of　the　L′3-type　lattice　via　a　sequential　mechanism　is　an　energetically　favorable　pathway　for　the　phase　transition　among　different　W2C　modifications.　Our　results　bring　a　deep　insight　into　the　understanding　of　the　stable　W2C　polymorphic　structures　and　would　be　very　helpful　to　identify　the　ordered　structures　of　highly　nonstoichiometric　tungsten　carbide　and　other　transition　metal　carbides　in　experiments.　©　2023　American　Chemical　Society.