Degradation of inorganic-organic perovskite films in ambient condition is a critical issue for the application of perovskite solar cells. Herein, it is the first time to report the catalytic role of H2O in the degradation of CH3NH3PbI3 perovskite by air of high humidity. It was found that H2O can catalyze the decomposition of CH3NH3PbI3 into CH3NH3I and PbI2, and then the crystal-to-amorphous phase transformation of the produced CH3NH3I. Copyright (C) 2016 John Wiley & Sons, Ltd.