In this work, an efficient and stable inverted planar perovskite solar cell (PSC) with water additive has been prepared in ambient air. Adding moderate water additive to CH3NH3PbI3-xClx precursor solution leads to growth in quality of CH3NH3PbI3-xClx film, because the boiling point of water is lower than that of N, N-dimethylformamide. The annealing temperature of the CH3NH3PbI3-xClx film is controlled to 105 degrees C (slightly higher than the boiling point of water) to obtain the dense perovskite film rapidly in ambient environment with 40% relative humidity. The highest power conversion efficiency (PCE) of PSC with water additive is 14.02%, higher than that (11.17%) of the reference device without water additive. In addition, the aqueous CH3NH3PbI3-xClx solution is transformed into the stable perovskite film through the method of rapid annealing. The stable CH3NH3PbI3-xClx center dot nH(2)O system can resist the decomposition of perovskite film and weaken iodide ion migration in PSCs, which enhance stability of the device under the atmospheric environment. After being stored in the air for 120 h, the PSC device with water additive still has an PCE of 12.01%, higher than that (1.64%) of the reference device.