Macroscopic　mechanical　properties　and　failure　mechanism　of　concrete　are　determined　by　the　microscopic　components.　And　the　interface　between　aggregate　and　cement　mortar　is　an　important　factor　that　affects　the　failure　path　and　macro-mechanical　properties　of　concrete.　The　interfacial　transition　zone　is　regarded　as　a　layer　of　near-field　mortar　with　a　high　porosity　different　from　the　far-field　mortar　matrix.　The　mechanical　properties　of　the　three-phase　composite　meso　elements　of　concrete　are　deduced　by　a　＇two-step　equivalent　method＇,　and　then　the　equivalent　constitutive　relationship　of　each　meso　element　is　obtained.　Based　on　the　meso　element　equivalent　model,　the　damage　process　and　the　macro-mechanical　properties　of　two-grade　concrete　are　investigated　under　the　loading　conditions　of　uniaxial　tension,　uniaxial　compression　and　bending.　And　the　influence　of　interface-phase　on　the　mechanical　properties　of　concrete　is　discussed.　Besides,　the　results　of　the　present　model　are　compared　with　the　ones　by　a　random　aggregate　model.　The　analysis　results　show　that　the　existence　of　interfacial　transition　zone　has　a　great　influence　on　the　macro　elastic　modulus,　strength　and　residual　strength　of　concrete,　which　should　not　be　neglected　when　analyzing　on　the　macro-mechanical　properties　and　damage　fracture　of　concrete.