Three isomeric acceptors FOM-1, FOM-2 and FOM-3 incorporating the fluorene center are designed and synthesized. FOM-1 and FOM-2 show C-shaped structures with two methoxy groups substituted on the different positions of the fluorene units, while FOM-3 adopts an S-shaped structure with the methoxy groups substituted on the asymmetric position of the fluorene ring. The effects of the molecular symmetry, the substituted positions of the methoxy groups on the photoelectric properties, and the molecular packing have been investigated. Organic solar cells based on the three acceptors demonstrate promising power conversion efficiencies (PCEs) of 14.34%, 15.10% and 15.02% for FOM-1, FOM-2 and FOM-3, respectively.