The　phase　structure　and　phase　transition　were　predicted　by　using　the　so-called　CALPHAD　approach.　And　the　mole　fractions　of　the　constituted　phases　in　Ti2AlNb-based　alloy　and　the　mole　fractions　of　the　constituted　elements　in　each　phase　were　obtained.　The　order-disorder　transitions　of　the　dominating　phase　in　Ti2AlNb-based　alloy,　denoted　as　O　phase,　were　investigated　by　combining　sublattice　model　with　first-principle　total-energy　calculations.　The　site　occupancy　fractions　of　the　alloying　elements　on　the　sublattices　were　calculated.　The　order-disorder　transformation　of　the　O　phase　belongs　to　the　second-order　transition　with　a　continuous　character.　The　predicted　results　agree　well　with　the　available　experimental　data.