Thermal-assisted blade-coating (TABC) technique has been widely applied for scalable production of perovskite solar cells (PSCs) while it is not applicable in ambient atmosphere due to the adverse impact of humility to perovskite films. Here we report a modified TABC method to achieve high quality perovskite films in a harsh ambient condition, which were prepared with lead acetate trihydrate (PbAc2-3H2O) as the lead source and followed with a low-temperature, short time annealing treatment. The as-prepared perovskite films were preferably orientated and had large grain domains up to 100 µm. Based on these films, the conversion efficiency of PSCs reaches 15.8±0.6%, nearly 40% boosting compared to that of PbAc2 sourced devices (11.4±1.0%). We found that small amount of hydrate water in PbAc2-3H2O lead source lead to dense and oriented nuclei at the blade-coating stage. The concomitant MAPbI3-xH2O on the surface and grain boundaries of perovskite films isolated humility in ambient and upon the annealing process, melted to form a quasi-liquid nutrition pool for the growing up of MAPbI3 grain domains by Ostwald ripening. As such, the final quality of perovskite films is greatly improved and so is the performance of PSCs. This work provides an original understanding on the role of hydrate water in the formation of perovskite films by the TABC method and the robust strategy present here contributes a significant progress towards scalable production of high efficiency PSCs in ambient condition.