This invention relates to a photovoltaic cell and its preparation method. The nano polycrystalline bio thin film photovoltaic cell as provided by the present invention is of layered structure, and the structure from top to bottom is: a top plate insulated and sealed layer, a conductive layer, a nano semiconductor layer, chromophoric molecular layer, an electrolyte polymer layer, a conductive catalyst layer, a conductive layer, a bottom plate insulated and sealed layer. The nano semiconductor layer is made of three metal oxide or metal sulfide, and the electrolyte polymer layer is made of CeCl3, gas SiO2 and LiI. The present invention also provides the preparation method of the nano polycrystalline bio thin film photovoltaic cell. The nano polycrystalline bio thin film photovoltaic cell mentioned in the present invention has higher photovoltaic conversion efficiency, lower cost, and can be applied in construction, family, community, factories and mines and electric net power supply.