Pure BaMgSiO4:Eu(2+) phosphor, prepared by a solid state reaction method under N2 atmosphere, exhibited a strong green emission at 500 nm and a weak emission at 405 nm. Heat treatment under NH3 atmosphere causes changes in the PL intensity: the green emission at 500 nm gradually decreases and completely disappears after heat treatment for 3 h, whereas a new blue emission peak, centered at 445 nm, appears and becomes very strong. The results of the analyses with electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) spectroscopy suggest that the heat treatment causes the generation of a large amount of oxygen vacancies. This resulted in the aforementioned color changes of the BaMgSiO4:Eu phosphor, which are confirmed by the results of DFT+U calculations. In particular, these calculations showed that Eu prefers to occupy Ba(3) sites, which are six coordinated to oxygen atoms. The emission at 500 nm was attributed to the 4f-5d transition energy of Eu in Ba(3) site, calculated as 2.54 eV. It was also shown that Eu 4f energy level decreases when oxygen is removed from the oxygen position adjacent to Eu, which results in a larger Eu 4f-5d transition energy and shorter wavelengths of emission peaks.