Codoping with Li+ is a prevalent strategy to improve the optical efficiency of luminescent materials, while the mechanisms of enhancement are still ambiguous. Herein, we delineate the major ways by which Li+ enhanced the emission of orthophosphate phosphor KMg4(PO4)(3):Eu2+ and quantify the relative contributions of each mechanism. Results from X-ray diffraction, scanning electron microscopy, and steady-state and time-resolved fluorescence spectroscopies show that the 3.8-fold increase in emission intensity caused by optimized Li+ doping was attributed to flux effect (similar to 30.84%), crystal-field splitting (similar to 5.30%), and a reduction in concentration quenching (similar to 63.86%), respectively. The as-synthesized materials also show excellent thermal stability and an increased internal quantum efficiency of 84.02% compared with 53.13% of nondoped phosphors. The white light emitting diodes employing KMg4(PO4)(3):Eu2+,Li+ as a blue-emitting component exhibit superior electroluminescence properties. The above results demonstrate that introducing Li+ ions can obviously enhance the luminescence efficiency of KMg4(PO4)(3):Eu2+ phosphor.