Constructing uniform, flat perovskite thin films is of paramount essence to achieve high performance and reproducible perovskite photovoltaic devices. However, the sequential deposition technique, which usually shows efficiencies over 17%, still failed to enable the formation of uniform, pinhole-free films by solution processing, whereas such high-quality films can be prepared within the vapor medium. Here we demonstrate that such high-quality film can also be realized upon a solution process via judicious choice of solvents, particularly by controlling of lead dissolution phenomenon. We found that alcohol solvents with appropriate molecular structures such as tert-butanol, can facilitate the in situ intercalation of ammonium cations into PbI2 matrix, and thus give flat mirror-like tetragonal perovskite film with resemble surface roughness to that from vapor assisted method. Planar heterojunction solar cells constructed with these solution-processed thin films yielded a high power conversion efficiency of 14.61%, extremely higher than that of conventional method under standard one sun conditions. This solvent-mediated strategy provides a new approach towards the goal of high-performance, low-cost and reproducible devices with simple solution processes.