Luminescent properties of inorganic optical materials activated by a variable valence ion depend tightly on the exact valence the ion finally exists.1-8 Once a lower valence ion doped optical material is targeted, a reducing atmosphere is usually necessary to promote the reduction reaction from higher to lower valence state during the preparation.7,9 In this work we have developed an approach by intentionally creating interstitial barium sites in a barium compound where bismuth can expectably reside, and found it can enable the reduction of Bi3+ to Bi2+ even when the sample is prepared in an oxidizing condition.9 The approach can generalize initially to Ba3(PO4)2∶Bi and afterwards Ba2P2O7∶Bi and Sr3(PO4)2∶Bi.It does not necessitate a reducing gas or an enclosed crystal structure to shelter an activator from surrounding.It is believed that this work could richen our knowledge on the technique to control bismuth valence, and meanwhile it could provide a general strategy in the future to prepare the optical materials doped with lower valence ion via a more secure,economical and convenient route.