The orthosilicate phosphors demonstrate great potential in the field of solid-state lighting, and the understanding of the structure-property relationships depending on their versatile polymorphs and chemical compositions is highly desirable. Here we report the structural phase transformation of Ca2-xSrxSiO4:Ce3+ phosphor by Sr2+ substituting for Ca2+ within 0 <= x < 2. The crystal structures of Ca2-xSrxSiO4:Ce3+ are divided into two groups, namely, beta phase (0 <= x < 0.15) and alpha' phase (0.18 <= x < 2), and the phase transition (beta -> alpha') mechanism originated from the controlled chemical compositions is revealed. Our findings verified that the phase transition Pnma (alpha'-phase) <-> P2(1)/n (beta-phase) can be ascribed to the second-order type, and Sr2+ ions in Ca2-xSrxSiO4 preferentially occupy the seven-coordinated Ca2+ sites rather than the eight-coordinated sites with increasing Sr2+ content, which was reflected from the Rietveld refinements and further clarified through the difference of the Ca-O bond length in the two polymorphs of Ca2SiO4. The emission peaks of Ce3+ shift from 417 to 433 nm in the composition range of 0 <= x < 0.8, and the difference in the decay curves can also verify the phase transformation process. Thermal quenching properties of selected Ca2-xSrxSiO4:Ce3+ samples were evaluated, and the results show that the integral emission intensities at 200 degrees C maintain >90% of that at room temperature suggesting superior properties for the application as white light-emitting diodes (w-LEDs) phosphors.