Several　Ce3+　centers　were　experimentally　found　in　Ce3+-doped　potassium　magnesium　fluoride　(KMgF3)　crystal　by　electron-spin　resonance　(ESR)　spectra,　but　there　was　no　definite　conclusion　about　the　charge-compensating　mechanism.　In　the　present　work,　we　carried　out　a　comprehensive　investigation　on　local　structures　around　Ce3+　centers　in　the　host,　based　on　a　combination　of　first-principles　calculations　and　phenomenological　analyses.　The　site　occupation　preference,　crystal-field　energy　levels　and　spectral　parameters　(e.g.　crystal-field　parameters,　and　g　factors)　for　Ce3+　ions　in　the　host　were　derived,　which　are　compatible　with　previously　measured　excitation　and　ESR　spectra.　Accordingly,　four　Ce3+　centers　(at　K+　sites)　observed　in　ESR　spectra　are　assigned　successfully.　In　particular,　the　Ce3+　center　previously　thought　to　be　associated　with　a　Mg2+　site　is　reassigned　to　a　K+　site,　in　agreement　with　the　results　of　defect　formation　energy　calculations.　The　presented　calculations　together　with　optical　and　ESR　spectroscopy　techniques　may　offer　cross　check　for　reliable　determination　of　impurity　sites　in　luminescent　materials.