There is a significant mismatch between the optical absorption properties and electrical transmission properties in organic active layer, this contradiction is regarded as a main reason for limiting the conversion efficiency of organic solar cells (OSC). Fortunately, plasmonic light trapping has emerged as an effective technology to overcome this mismatch. Metal gratings are embedded in OSCs as bottom electrodes, and an OSC structure based on plasmonic grating electrode is proposed. Furthermore, optical field distributions in OSCs are numerically investigated by finite difference in time domain, and the plasmonic enhancement effects of metal gratings are analyzed. The results show plasmonic gratings have excellent light trapping ability and response to incident angle, so there is obvious light absorption enhancement in active layer.