Eu2+-activated β-Ca3(PO4)2-type phosphors have attracted significant experimental interest for applications in solid-state lighting because of the presence of multiple cation sites, which is highly desirable for site engineering of activator emission. However, the site occupation and associated spectral assignment of dopant Eu2+, and hence the mechanism behind the site-regulated emission tuning, still remain elusive. Herein, we perform a systematic first-principles study on Eu2+-doped Ca3(PO4)2, Ca10M(PO4)7 (M = Li, Na, K), and Ca3(PO4)2:Mg by combining density functional theory and multiconfigurational ab initio calculations. The results show that, among the isovalent EuCa substitutions in Ca3(PO4)2, the occurrence probability correlates positively with the size of the substituted site, which is, nevertheless, weakened by the incorporation of codopant Mg2+ ions. In the presence of aliovalent EuM substitutions as in Ca10M(PO4)7, the site-size-controlled preference is neutralized by the requirement for charge compensation, and the effect becomes more pronounced with an increase of the M+ ionic size. On this basis, the emission spectra of the phosphors are interpreted with respect to the substituted sites and the mechanism behind the composition dependence of the emission color is consistently elucidated, as is also verified by a comparison between the calculated 4f → 5d transition energies and experimental excitation spectra. Our results provide a new perspective on the site preference of Eu2+ in β-Ca3(PO4)2-type compounds and may also serve as a theoretical guideline on the structure-property relationship for the design of other Eu2+-activated phosphors.