The paper reported the synthesis of mixed-valence Eu-doped Mg3Ca3(PO4)(4) by solid-state reaction in the air atmosphere. The luminescence measurements indicated that the obtained phosphors exhibit a broad bluish-green fluorescence of Eu2+ and a sharp orange-red emission of Eu3+ excited by 395 nm. The abnormal reduction mechanism could be explained by a charge compensation model. A tunable luminescence was realized based on the reduction of Eu3+ to Eu2+ changed along with increasing the sintering time. The reduction of Eu3+ to Eu2+ in Mg3Ca3(PO4)(4) host was effected by the oxygen vacancy defects when the samples sintered in different atmosphere and sintered for different lengths of time, and the oxygen vacancies which act as the electron traps were investigated with thermoluminescence. All of the results almost indicated the oxygen vacancy could weaken the reduction of Eu3+ to Eu2+. Our investigation of Mg3Ca3(PO4)(4):Eu could provide a practical basis and theoretical basis to study the effect of oxygen vacancy on the emission of Eu ions.