Humid environment plays a vital role in affecting the performance stability of the organic metal halide perovskite solar cells. Therefore, in situ monitoring the micro-structural evolution of perovskite film in real time will help to reveal the micro-mechanism for the device performance decay induced by humidity. A device providing a controllable humid environment has been set up at X-ray diffraction beamline of Shanghai Synchrotron Radiation Facility, which is used to monitor in situ the perovskite film micro-structural evolution in real time in a humid environment by using grazing incidence X-ray diffraction(GIXRD). After a perovskite film is exposed to the environment with a relative humidity of 60%±2%, a new component emerges near the perovskite(110) diffraction peak in the early stage of the exposure, which is observed for the first time. This new component is attributed to the perovskite intermediate phase structure transformed from the gradual degradation of the perovskite crystalline. Meanwhile, UV-Vis absorption measurements show that humidity causes the absorption of the film to decrease slightly with the blue shift of the absorption edge at ~770 nm, which indicates a reduced amount of perovskite crystalline or a decrease of perovskite crystallinity. Scan electron microscope further demonstrates that the film after the humid exposure presents a worse morphology with a lower coverage, bigger pores, and larger voids between crystalline than the pristine film. The current-voltage(J-V) measurements of the solar cells fabricated on the perovskite films before and after the humid exposure show that both the filling factor and the power conversion efficiencydecrease by over 30% due to the humidity. The present work demonstrates that the close relationship between the device performance and the perovskite film microstructure as well as their morphologies can be studied very well by in-situ synchrotron based characterization technique. The present study could lay a good foundation for the understanding of the degradation mechanism for the organic metal halide perovskites.