On　a　mesoscopic　scale,　concrete　is　considered　to　be　a　heterogeneous　composite　material　which　is　composed　mainly　of　cement　mortar,　aggregate,　an　interface　transition　zone　(ITZ),　and　pores.　The　mesoscopic　model　of　concrete　was　established　by　applying　digital　image　processing　(DIP).　To　study　the　impact　of　porosity　on　the　compressive　strength　of　concrete,　the　pore　equivalent　model　was　introduced　to　build　concrete　with　different　pore　contents.　The　base　force　element　method　(BFEM)　based　on　the　complementary　energy　principle　was　used　in　the　numerical　simulation　of　concrete.　This　paper　investigated　the　effect　of　porosity　on　the　strength　of　concrete　under　uniaxial　compression.　The　results　showed　that　the　mesoscopic　model　of　concrete　based　on　DIP　technology　can　better　simulate　the　strength　test　and　failure　mechanism　of　concrete　by　using　BFEM.　As　the　porosity　in　the　ITZ　increases,　the　peak　strength　of　concrete　under　compression　decreases.　The　compressive　strength　of　concrete　also　decreases　with　the　growth　of　porosity　in　the　mortar　matrix.　The　ITZ　porosity　has　a　greater　effect　on　the　compressive　strength　of　concrete　than　the　mortar　porosity.