Organolead-halide-perovskite-based solar cells have recently attracted significant attention due to excellent photovoltaic performance and low cost. To improve the conversion efficiency, lots of efforts have been devoted to optimize chemical components and the film fabrication. However, the kinetics of solutes in solution and its effect on the film quality are less understood. Our results reveal that aging of the precursor solution containing PbI2 and CH3NH3I facilitated crystallization and resulted in a more uniform film. Further analysis indicates that in solution PbI2 flakes were gradually dissociating into small oligomers, and then form complex with CH3NH3I.