Organic solar cells (OSCs) based on small molecular donor/polymer acceptor (M-D/P-A) blends have the advantage of excellent thermal stability, but they suffer from low photovoltaic efficiency because of the large size phase separation morphology of the active layer. Here, we systemically studied the effects of molecular weight on the active layer morphology and device performance of M-D/P-A-type OSCs. When the molecular weight is low, the polymer acceptor shows weak aggregation in solution and poor crystallinity in film, which allows the small molecular donor form large crystalline domains in M-D/P-A blends. In contrast, high molecular weight leads to strong aggregation of polymer chains in solution, which not only enhances crystallinity of the polymer acceptor but also suppresses crystallization of the small molecular donor. As a result, the M-D/P-A-type OSC exhibits small-size phase separation in the active layer and shows much enhanced photovoltaic performance with a power conversion efficiency of 6.4%, which is among the highest reported for M-D/P-A-type OSCs. These results disclose the great importance of high molecular weight for efficient M-D/P-A-type OSCs.