The　second-order　nonlinear-polarization　originated　from　the　interaction　between　thin-film　materials　with　second-order　nonlinear　susceptibility　(chi((2)))　and　high-power　laser　is　essential　for　integrated　optics　and　photonics.　In　this　work,　strong　second-order　nonlinear-polarization　was　found　in　a-axis　oriented　Zn1-xMgxO　(ZnMgO)　epitaxial　thin-films　with　Li　incorporation,　which　were　deposited　by　radio-frequency　magnetron　sputtering.　Mg　incorporation　(x　＞　0.3)　causes　a　sharp　fall　in　the　matrix　element　chi(33)　of　chi((2))　tensor,　although　it　widens　optical　bandgap　(E-opt).　In　contrast,　moderate　Li　incorporation　significantly　improves　chi(33)　and　resistance　to　high-power　laser　pulses　with　a　little　influence　on　E-opt.　In　particular,　a　Zn0.67Mg0.33O:Li　[Li/(Zn　+　Mg　+　Li)　=　0.07]　thin-film　shows　a　|chi(33)|　of　36.1　pm　V-1　under　a　peak　power　density　(E-p)　of　81.2　GW　cm(-2),　a　resistance　to　laser　pulses　with　E-p　of　up　to　124.9　GW　cm(-2),　and　an　E-opt　of　3.95　eV.　Compared　to　that　of　ZnO,　these　parameters　increase　by　37.8%,　53.4%,　and　18.6%,　respectively.　Specially,　the　Zn0.67Mg0.33O:Li　shows　higher　radiation　resistance　than　a　Mg-doped　LiNbO3　crystal　with　a　comparable　E-opt.　First-principle　calculations　reveal　the　Li　occupation　at　octahedral　interstitial　sites　of　wurtzite　ZnO　enhances　radiation　resistance　by　improving　structural　stability.　X-ray　photoelectron　spectroscopy　characterizations　suggest　moderate　Li　incorporation　increases　chi(33)　via　enhancing　electronic　polarization.　These　findings　uncover　the　close　relationship　between　the　octahedra　interstitial　defects　in　wurtzite　ZnMgO　and　its　nonlinear-polarization　behavior　under　the　optical　frequency　electric　field　of　high-power　laser.