Non-fullerene organic solar cell (NFOSC) has attracted tremendous attention due to their great potential for commercial applications. To improve its power conversion efficiency (PCE), generally, sequential solution deposition (SSD) methods have been employed to construct the graded vertical phase separation (VPS) of the bulk-heterojunction (BHJ) active layer for efficient exciton separation and charge transition. However, a variety of orthogonal solvents used in the SSD may lead to the unpredicted change in the BHJ morphology and introduce additional defects inside the BHJ bulk thus complicate the fabrication process. Here, a simple oscillating stratification preprocessing (OSP) is developed to facilitate the formation of graded VPS among the BHJ layer. As a result, a significant improvement is obtained in PCE from 10.96% to 12.03%, which is the highest value reported among PBDB-T: ITIC based NFOSC.