Silver nanoparticles (Ag NPs) are synthesized with chemical method, which are introduced into the traditional organic photovoltaic (OPV) structure. The experimental results show that both the optical and photoelectric properties are enhanced because of localized surface plasmon (LSP) effects of Ag NPs. The advantage of adding Ag NPs behind active layer in incident direction is discussed. We believe this route can avoid absorption shadow and enhance the reusing of transmitted light of active layer. The average short-circuit current (J_SC) of the optimum device can be raised from 9.23 mA/cm² to 10.84 mA/cm² , and the energy converting efficiency (PCE) can be raised from 3.22% to 3.85%.