Bulk-heterojunction organic solar cells generate electrical power from light absorption in the active layer and consist of a mixture of electron donor and acceptor materials. The donor/acceptor ratio is one of the critical parameters that determines the electrical characteristics of bulk-heterojunction solar cells. Therefore, the optimal ratio of the donor/acceptor must be determined to achieve maximum efficiency in bulk-heterojunction solar cells. In this work, modified spray- or mist-coating methods are applied to control the donor/acceptor ratio systematically and continuously. The feasibility of this processing method for polymer-ratio screening is investigated by monitoring the morphological and optical properties of polymer solar cells based on poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PCBM). Finally, the optimum ratio of P3HT/PCBM is obtained by increasing the ratio of P3HT/PCBM gradually. This study provides a strong and practical screening method to define the critical mixture ratio among the enormous number of polymer–fullerene combinations.