<p>Printable mesoscopic perovskite solar cells (p-MPSCs) have received widespread attention in recent years due to their excellent stability and other characteristics. However, the three-layer mesoporous structure of p-MPSCs is a double-edged sword. While providing natural barrier for perovskite, it also has a negative impact on crystallization of perovskite, making it prone to the formation of more defects. The non-radiative recombination induced by defects deteriorates the effective transport and extraction of charge carriers in the mesopores, ultimately limiting the improvement of the power conversion efficiency (PCE) of p-MPSCs. There are many natural small molecules in nature that contain multi-functional groups, which can regulate the crystallization process and passivate the defects of perovskite by forming intermediates with perovskite. This work is the first to add natural small molecule cinnamic acid (CA) to Cs_0.05MA_0.2FA_0.8Pb_1.05I_3.05Cl_0.1 perovskite to improve the performance of p-MPSCs. Fourier transform infrared spectroscopy verifies the chelation effect between the carboxyl group of CA and undercoordinated Pb²⁺ of perovskite. Scanning electron microscopy reveals that the introduction of CA increases the grain size of perovskite and makes the mesopores denser. Further, the space charge limited current proves that CA effectively reduces the defect density of perovskite. Finally, the PCE of p-MPSCs optimized by CA increased from 17.67% before modification to 18.55% for the champion. In addition, CA-optimized p-MPSCs maintained 85% of their initial PCE after being stored in dark with 30 ± 5°C and 40 ± 5% RH for 1536 h without encapsulation. This improvement is mainly due to the enhanced crystallinity induced by CA. This study developed a simple method based on natural molecule to regulate the crystallization of perovskite in mesopores and reduce defects, providing new ideas for preparing efficient and stable p-MPSCs based on the green defect passivation approach.</p>