A series of novel oxynitride apatite structure phosphors, La_10−xSr_x(Si_6−xP_xO₂₂N₂)O₂ (x = 0, 1, 2, 3, 4):Eu²⁺, with an ([Si/P][O/N]₄) structure were engineered and synthesised via a solid-state reaction. Crystal structure information, especially changes in cell parameters and average bond lengths from the luminescence centres to the ligands, were refined using the Rietveld method. Compared with pure oxide phosphors with a similar apatite structure, the Eu ions can more readily be reduced with increasing N³⁻ or P⁵⁺ concentration due to the additional reducing potential of N³⁻ ions and the extra adjustment patterns for stabilisation of Eu²⁺ ions. With increasing Eu²⁺ concentration, the intensity of different samples exhibits peculiar and interesting variations, which is related to the different reducing capacity in different samples and the energy absorption function of Eu³⁺ ions. All phosphors show good excitation ranging from 250 nm to 450 nm, and the broadband emission spectra have a large shift, with the peak shifting from 464 nm to 520 nm. The possible reasons and mechanisms for the variations of the excitation and emission spectra and thermal quenching properties have been studied in detail. The results of Commission International de L'Eclairage chromaticity coordinates indicate that the emission colours can be tuned in a large region from blue (0.1625, 0.1864) to green (0.2788, 0.5735). These results reveal the correlations between structure and properties of luminescent materials and provide a practical basis to engineer and develop Eu²⁺-doped novel phosphors.