Widegap　ZnMgO　thin　films　hold　great　potential　for　applications　to　the　high-efficiency　generation　of　deep　UV　laser　via　frequency　conversion,　because　of　low　light　propagation　loss　and　reversible　polarization.　In　this　work,　we　studied　the　effect　of　Li　concentration　on　the　second　harmonic　generation　(SHG)　from　a-axis　oriented　ZnMgO　thin　films　with　an　optical　bandgap　of　about　3.96　eV.　The　SHG　intensity　per　unit　thickness　from　the　ZnMgO　thin　film　with　a　small　Li　concentration　is　approximately　10.7　times　larger　than　the　one　without　Li　incorporation.　The　Li　incorporation　obviously　enlarges　interplanar　spacing　of　(　1120　)　plane　and　reduces　the　leakage　current　by　three　orders　of　magnitude.　X-ray　photoelectron　spectroscopy　measurements　suggest　the　Li　incorporation　enhances　SHG　via　enhancing　electronic　polarization.　These　findings　uncover　defect　complexes　based　on　Li　interstitials　in　oxygen　octahedron　play　a　very　important　role　in　enhancing　the　nonlinear　polarization　of　widegap　ZnMgO　thin　films　under　optical　frequency　electric　field.