An organic solar cell comprises: a substrate, a cathode, an electronic collection layer, an active layer, a hole collecting layer and an anode. The electronic collection layer is formed by a polar organic small molecular film. The polar organic small molecular film can change surface potential of a material under irradiation of visible light. The cathode and the electronic collection layer are modulated so that ohmic contact can be formed between the cathode and the electronic collection layer. Open circuit voltage can be increased. The interface potential can be decreased. Series resistance of the cell is reduced and electron transmission is raised. An efficiency of the organic solar cell can be increased. Simultaneously, the invention also provides a preparation method of the solar cell.