We demonstrate how the annealing determines the structural evolution of CH3NH3PbI3-xClx perovskite and eventually its devices performance.In situ real-time synchrotron-based grazing incidence X-ray diffraction(GIXRD)clearly illustrated a step-by-step structural transformation from organic-inorganic hybrid materials into CH3NH3PbI3 perovskite crystal during the annealing in a flowing N2 atmosphere,which facilities the formation of high-quality perovskite crystallines without degradation.Notably,the perovskite crystalline grains were found to increase with the annealing time and its ordered crystal orientation along in-plane direction also enhanced obviously as the extension of annealing time.The relationship between the step-by-step crystal structure transitions process of precursor-to-perovskite and the corresponding performance of the mixed halide perovskite solar cells was carefully studied,and the fabricated devices using PEDOT:PSS as the hole conductor were found with a highest power conversion efficiency(PCE)up to 14.36%.Our study not only illustrates clearly the decisive roles of annealing in the formation ofperovskite,but also demonstrates an effective approach to fabricate high quality perovskite films for high performance solar cells.