An unexpected spectrum blue shift of the yellow emission was observed for solid solution phosphors Ba2-xSrxMg(PO4)(2):Eu2+ (x=0-1.5), when bigger ions Ba2+ were substituted by smaller ions Sr2+ in Ba2Mg(PO4)(2) lattices. The solid solution phosphors were prepared by a solid-state reaction to clarify the local sites environment of activators Eu2+ and tune the anomalous long-wavelength yellow emission. BaSrMg(PO4)(2) phase was identified to be isostructural with Ba2Mg(PO4)(2) by Rietveld refinements. DFT calculation and photoluminescence show that the activator ions Eu2+ in BaSrMg(PO4)(2) occupy Sr2+ and Ba2+ sites at an equal probability and both generate an anomalous yellow emission. The yellow emission from Ba2-xSrxMg(PO4)(2):Eu2+ was gradually blue shift with increasing Sr2+ substitution concentration x from 0.1 to 1. Such unusual blue shift is interpreted based on the evolution of crystal structure parameters due to Sr2+ substitution, and subsequently, a site environmental expansion mechanism is proposed. The proposed mechanism could serve as a general model to reveal the underlying factors for spectrum shift caused by cation substitution and contribute to design new solid solution phosphors.