An olivine-type Eu2+-activated NaMgPO4 (NMP:Eu2+) material was previously discovered as a new red-emitting oxide phosphor with a high quantum efficiency under blue light excitation; however, the use of a special and nonuniversal optical system of an arc-imaging furnace was necessary for its synthesis. To realize the synthesis of NMP:Eu2+ using a conventional tubular furnace, in this work, we employed a new synthetic strategy of rapid quenching from a high temperature, which imitates the synthesis process using the arc-imaging furnace. As a result, we succeeded in stabilizing the olivine structure in the NMP:Eu2+ phosphor. Thermogravimetric-differential thermal analysis (TG-DTA) revealed the presence of the polymorphism of glaserite (low-temperature phase) and olivine (high-temperature phase) phases in the NaMgPO4:0.025Eu2+ composition, and the stabilized region of the olivine phase was found to be around 1000 °C, which indicated that the rapid quenching process effectively acted as an olivine phase stabilizer. According to X-ray diffraction (XRD) measurements, the NMP:Eu2+ phosphor samples have the olivine structure as the main phase. Additionally, the photoluminescence (PL) intensity of the phosphor was 1.3 times that of the previous NMP:Eu2+ phosphor prepared using the arc-imaging furnace. The internal quantum efficiency of the NMP:Eu2+ phosphor was found to be 72.3%, which is the highest among the known Eu2+-activated red-emitting oxide phosphors. Moreover, other properties relevant to light-emitting diode (LED) phosphors, such as the chemical composition, chemical state of Eu, particle morphology, PL lifetime, thermal quenching properties, humidity resistance and electroluminescence (EL) properties of a fabricated phosphor-converted (pc)-LED, were investigated in detail.