The optical absorption properties of Ag-BaO composite thin film with excess barium were measured while a vertical electric field was applied on the film surface.Total two absorption peaks were observed in the visible and near-infrared region,and only the peak that lies in the near-infrared region was dependent on the applied electric filed,i.e.,the absorption intensity was decreased with the increasing electric field.The dominant peak in the visible region is attributed to the surface plasmons resonance of Ag ultrafine particles embedded into the BaO semiconductor matrix.However,the weaker one in the near-infraed region is considered to arise from the transition of bound electrons at impurity energy level of the BaO semiconductor matrix,and such an impurity energy level is related to the negative ion vacancies caused by the excess braium in the BaO crystal.When the external surface electric field was applied,some of the impurities in the BaO Semiconductor matrix were ionized,and which further resulted in a decrease of the density of bound electrons at the impurity energy level.So that,part of the optical absorption owing to the impurity energy level was decreased with the enhanced electric field.