Organic photovoltaic cells (OPVs) are fabricated with a polymer donor PM7 and a nonfullerene acceptor IT-4Cl; the morphology of active layers is optimized by employing upside-down solvent vapor annealing (UD-SVA) method with different annealing solvents. The OPVs with CS2 as annealing solvent exhibit optimized power conversion efficiency (PCE) of 13.76%, with simultaneously increased short-circuit current density (J(SC)) of 20.53 mA cm(-2) and fill factor (FF) of 77.05%. More than 15% PCE improvement can be achieved by employing CS2 UD-SVA treatment, which should be attributed to slightly enhanced photon harvesting, efficient exciton separation, charge transport, and collection, resulting from the well-developed morphology of active layer. Moreover, the PM7:IT-4Cl-based OPVs with CS2 as annealing solvent still can maintain PCE more than 13% in a wide treatment time range from 20 to 90 seconds. This work demonstrated that UD-SVA has great potential in improving the performance of nonfullerene OPVs.