Two different Sr2SiO4:Dy3+ phosphors were successfully synthesized at 1000°C for 4 h using NH4Cl or SrCl2 as the flux, respectively. The luminescence intensity of Sr2SiO4:Dy3+ was enhanced 60 times by the addition of 5 wt% NH4Cl. The phase evolution and photoluminescence (PL) properties of the phosphors were investigated by the calculation of phase equilibrium and the analysis of TG/DSC, XRD, SEM, and PL. The role of NH4Cl was proved to be attributed to the formation of SrCl2 at ~300°C. The flux NH4Cl or SrCl2 lowered the decomposition temperature of carbonate and promoted the preferential formation of α′ phase in the solid-state reaction through accelerating the kinetics of the formation of Sr2SiO4 by enhancing diffusion with more liquid phase.