Lanthanide downconversion nanoparticles (DCNPs) have huge potential in biosensing and imaging applications in the NIR-II window. However, DCNPs inherently suffer from low quantum efficiency, due to low absorption cross-section and the restricted doping concentration of lanthanide ions. In this work, a combined strategy for downconversion luminescence in the NIR-II window is investigated by the integration of Ce3+ ions into the conventional NaYF4: Yb3+, Er3+ DCNPs and incorporation of periodic silver hole-cap coupled Nanoarrays (Ag-HCNAs) simultaneously. Over two orders of magnitude, luminescence enhancement is achieved by the combination of optimized Ce3+ doping and plasmonic effects, compared to NaYF4: Yb3+, Er3+ DCNPs immobilized on the glass substrate. Moreover, 3D Finite-Difference Time-Domain (FDTD) simulations and time-resolved luminescence measurements are combined to gain important insights into the mechanism of downconversion luminescence enhancement. The results show that there is a large electric field enhancement between the Ag nanoholes and the Ag hemisphere cap at 980 nm (excitation enhancement), while the lifetime shortening at 1525 nm revealed an increased radiative decay rate and enhanced quantum yield (emission rate enhancement). The strategy for downconversion luminescence enhancement demonstrated in this work holds a significant potential for advancing the next generation biosensing and bioimaging based on DCNPs in the NIR-II window.