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Due to Eu2+ and Eu3+ ions often co-exist in the crystal lattices, the luminous intensity of some phosphors is limited greatly. In this work, we propose a crystal-site engineering approach to reduce Eu3+ to Eu2+ in the KBaYSi 2 O 7 lattice through substitution of Ba2+ by Ca2+ or Sr2+. The mechanism between the local environmental regulation of the crystal structure around Eu2+ as well as the luminous intensity and thermal stability of the phosphor was systematically explored. The results of XRD, Rietveld refinement and EDX confirmed that Ca2+ and Sr2+ are dissolved into the crystal lattice effectively without changing the main phase structure. As a result, the luminous intensity of samples doped Ca2+ and Sr2+ increased by 39.7% and 41.2%, respectively. The mechanisms for such improvement is ascribed to the reduction of Eu3+ into Eu2+ under the lager volume of [EuO 9 ] 2 polyhedron. The treated samples exhibited relatively superior thermal stability with I 150°C > 70%, indicating that doping with Ca2+ or Sr2+ have no effect on the thermal stability. A pc-WLED device fabricated by combining (Ba,Sr) 2 SiO 4 :Eu2+, (Ca,Sr)AlSiN 3 :Eu2+, BaSi 2 O 2 N 2 :Eu2+ and KBa 0.86 Ca 0.1 YSi 2 O 7 :Eu2+ with 380 nm chip exhibits CCT (5010 K) and high CRI (96.6). These results indicate that this phosphor has potential for applications in full-spectrum warm white lighting. • The luminous intensity of KBa 0.96 YSi 2 O 7 :Eu2+ doped Ca2+ and Sr2+ increased by 39.7% and 41.2%, respectively. • The effective concentration of Eu2+ ions was enhanced by structural modulation. • A white-LED device with an excellent CRI (Ra = 96.6) and low CCT (5010 K) was obtained. [ABSTRACT FROM AUTHOR]