Definite emission color from rare-earth Eu2+ cannot be guaranteed in distinct hosts because its spectrum behavior is strongly dependent on surrounding microenvironment. Herein, we propose a strategy of heterostructure polyhedron BO3-PO4 substitution that can realize customizable and even predictable Eu2+ emission. Taking Sr3La(PO4)3:Eu2+ blue phosphor as host, we prepared a series of BO3-PO4 substitution-designed Sr3La(PO4)3−x(BO3)x:Eu2+ (SLP3−xBx:Eu2+) phosphors via solid-state reaction. Structural and spectral analyses demonstrate that substitution of PO4 with BO3 unit drives Eu2+ to migrate from original occupied Sr sites to unoccupied six-coordinated La sites, bringing out a brand-new broadband yellow-emitting peak at 530 nm, enabling an efficient spectrum tailoring from initial blue emission at 420 nm to white-light and then yellow. Strikingly, we find that the resultant Eu2+ spectrum behavior in as-prepared SLP3−xBx:Eu2+ and Eu2+-doped other borophosphate phosphors is highly similar (although they have different microenvironments). Such exciting findings indicate that proposed BO3-PO4 substitution-strategy possesses an ability of predicting emission by modulating Eu2+ site-selective occupation. Utilizing SLP3−xBx:Eu2+ (x = 0.1 and 0.4) phosphors, we fabricated optical temperature sensor and white LED prototypes, showcasing remarkable temperature sensitivity of Sr = 1.1%/K and good color rendering index (CRI) of 83. This work may aid the discovery of novel functional materials with specific, desirable physicochemical properties.