Surface quality of the perovskite layer plays an important role in the determination of the device performance. This study presents an effective method to optimize the surface morphology for the lead-free formamidinium tin iodide (FASnI(3)) perovskite solar cells. Poly(alpha-methylstyrene) (PAMS) was applied as polymer additive into the diethyl ether at different blend concentration. The addition of PAMS can apparently reduce the pinholes at the surface of the FASnI(3) film without ruining the interior crystallinity of the bulk FASnI(3) layer. Therefore the non-radiative carrier traps can be effectively suppressed. The surface of the FAsnI(3) film becomes more hydrophobic which facilitates the consequent spin-coating of the acceptor material solution. The modification of FASnI(3) film's wettability is favorable of the photo-induced carriers' extraction and transportation. As a result, the power conversion efficiency of the PAMS-modified devices is greatly improved.