Polycrystalline Bi2-xO2Se ceramics were synthesized by spark plasma sintering process. Their thermoelectric properties were evaluated from 300 to 773K. All the samples are layered structure with a tetragonal phase. The introduction of Bi deficiencies will cause the orientation alignment and change ofeffective mass. As a result, a significant enhancement of thermoelectric performance was achieved. The maximum of Seebeck coefficient is -568.8V/K for Bi1.9O2Se at 773K, much larger than -445.6V/K for pristine Bi2O2Se. Featured with very low thermal conductivity [similar to 0.6W(mK)(-1)] and an optimized electrical conductivity, ZT at 773K is significantly increased from 0.05 for pristine Bi2O2Se to 0.12 for Bi1.9O2Se by introducing Bi deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.