The　enhancement　of　mechanical　properties　of　concrete　meso-components　and　the　interaction　caused　by　non-uniform　deformation　as　well　as　phase　change　can　cause　significant　changes　in　the　macro-mechanical　performances　of　concrete　at　low　temperatures.　Based　on　the　action　mechanism　of　the　above　low-temperature　effect,　this　paper　established　a　thermal-mechanical　sequential　coupled　simulation　method　with　explicit　modelling　of　pore　ice　at　the　mesoscale　level　to　quantitatively　investigate　the　direct　tensile　failures　and　the　corresponding　size　effect　of　concrete　with　four　structural　sizes　(D75,　D150,　D225　and　D300)　and　three　moisture　contents　(2.0　%,　4.0　%　and　6.0　%)　at　different　temperatures　(20,　-30,　-60　and　-90　degrees　C),　in　term　of　failure　mode,　deformation　curve,　peak　strength　and　residual　strength.　The　numerical　results　show　that　the　direct　tensile　peak　strength　performs　an　obvious　low-temperature　enhancement　effect　due　to　the　more　damaged　aggregates　and　more　areas　being　in　a　state　of　multi-axial　stress　caused　by　low-temperature　non-uniform　stress　field.　However,　with　the　decreasing　temperature,　the　residual　strength　shows　a　decrease　trend　and　the　trend　slows　down　with　the　increasing　moisture　content.　Besides,　as　the　temperature　drops　from　20　degrees　C　to　-90　degrees　C,　both　the　size　effects　on　direct　tensile　peak　strength　and　residual　strength　are　strengthened　(with　the　increase　approaches　nearly　200　%　for　peak　strength　while　33　%　for　residual　strength).　Finally,　a　modified　size　effect　theoretical　model　was　developed　considering　the　quantitative　coupling　effects　of　low　temperature　and　moisture　content.　The　present　research　results　can　provide　a　reference　for　the　performance　evaluation　and　safe　design　of　large-sized　concrete　exposed　to　low-temperature　environments.