Zinc oxide (ZnO) is used as an electron transport layer (ETL) in organic solar cells (OSCs) owing to its excellent performance, such as good light transmission and high conductivity, but its surface defects and the high annealing temperature limit its development in flexible organic solar cells (FOSCs). Here, we adopt a new method to prepare ZnO which reduce annealing temperature. Then we apply syringic acid (SA) to modify the surface of ZnO film. It is found that SA can effectively passivate the surface defects of ZnO and optimize surface morphology, as well as reduce the work function (WF) of ZnO resulting in an improvement of the charge transport of FOSCs. Ultimately，The power conversion efficiency (PCE) of PM6:Y6-based FOSCs increased from 12.01% to 14.26%. Meanwhile, the mechanical stability of the cells after SA modification is significantly enhanced.