A single-composition emission-tunable phosphor BaY2Si3O10:0.01Bi3+,mEu3+ has been prepared by the conventional high temperature solid-state reaction. The X-ray diffraction (XRD), luminescent properties and lifetimes are used to characterize the obtained phosphor samples. Upon near UV light excitation, BaY2Si3O10:0.01Bi3+ and BaY2Si3O10:0.13Eu3+ phosphors show the typical emissions of Bi3+ (3P1 → 1S0, blue and green) and Eu3+ (5D0 → 7F2, red), respectively. The spectral analysis indicates that there exist two different Bi3+ luminescent centers (Bi(I) and Bi(II)) due to Bi3+ ions occupying different crystallographic sites in the BaY2Si3O10 host lattice, and the energy transfer between two types of Bi3+ ion luminescent centers was observed. The significant spectral overlap between the emission spectrum of Bi3+ ions and the excitation spectrum of Eu3+ ions implies the occurrence of energy transfer from Bi3+ to Eu3+ and the efficient energy transfer from Bi3+ to Eu3+ has been investigated in detail. The mechanism of energy transfer from Bi3+ to Eu3+ has been discussed by using Dexter's theory, demonstrating to be a resonance type via dipole–quadrupole interaction. Moreover, the critical distance for energy transfer from Bi3+ to Eu3+ has been calculated to be 14.38 Å using the concentration quenching method. The white light emission of BaY2Si3O10:0.01Bi3+,mEu3+ phosphors can be realized by systematically adjusting the activator dopant concentrations and utilizing the energy transfer under a near UV light pump. The quantum efficiency of BaY2Si3O10:0.01Bi3+,mEu3+ phosphors is measured, and the samples exhibit excellent quantum efficiency. The Commission International de I'Eclairage (CIE) chromaticity coordinates, correlated color temperature (CCT) and temperature dependence emissions are also discussed in detail. The energy transfer efficiency between Bi(I) and Bi(II) changes with increasing temperature due to a phonon-assisted process. All the results indicate that the white light emitting single-composition phosphor BaY2Si3O10:0.01Bi3+,mEu3+ has potential applications in white light LEDs.