The inverted perovskite solar cell fabricated using a two-step method exhibited the highest FF of 0.85 and good efficiency of 18% based on CH3NH3PbI3. A small amount of H2O was added into PbI2/DMF to make a homogenous precursor solution. A high quality PbI2 film with full coverage was formed on a PEDOT:PSS surface by spin coating of the homogeneous PbI2 precursor solution. The perovskite film fabricated from the high quality PbI2 film is highly pure, smooth and very dense even without any pinhole. The champion cell achieves a remarkable fill factor (FF) of 0.85, which is the highest value reported in perovskite solar cells. The FF value is also very reproducible with less than 10% deviation for 50 cells. The cell exhibits no current hysteresis and is stable under both dark and illumination conditions in dry and inert atmospheres. The results not only provide a strategy to fabricate high efficiency inverted perovskite solar cells but also reveal how the water additive in the PbI2/DMF solution may affect the properties of PbI2 and therefore the perovskite film prepared using the two-step method and the overall photovoltaic performance of the corresponding inverted solar cell.