Despite the exceptional performance of hybrid perovskites in photovoltaics, their susceptibility to ambient factors, particularly humidity, gives rise to the well-recognized stability issue. In the present work, microstripes of CH3NH3PbI3 are fabricated on flexible substrates, and they exhibit much larger response to relative humidity (RH) levels than continuous films and single crystals. The resistance of microstripes decreases by four orders of magnitude on changing the RH level from 10 to 95%. Fast response and recovery time of 100 and 500 ms, respectively, are recorded. Because bulk diffusion and defect trapping are much slower processes, our result indicates a surface-dictated mechanism related to hydrate formation and electron donation. In addition, water uptake behavior of perovskites is studied for the first time, which correlates well with the resistance decrease of the CH3NH3PbI3 microstripes. Furthermore, we report that the photoresponse decreases with increasing humidity, and at the 85% RH level, the perovskite device is not photoresponsive anymore. Our work underscores patterned structures as a new platform to investigate the interaction of hybrid perovskites with ambient factors and reveals the importance of the humidity effect on optoelectronic performance.