The utilization of high-purity materials is crucial to preserve the intrinsic properties of perovskites. However, the use is limited by the presence of water molecules in a perovskite solution, which may form impurities. Therefore, H2O should be removed from the solution. To improve this issue, herein, a moisture trap (MT) for selective moisture adsorption was introduced based on comparison of the diameters of molecules (solvent and H2O) in a perovskite solvent. In this context, a mechanism of perovskite degradation with moisture and the MT introduction effect were proposed as for solving the problem. Specifically, we investigated the degree of hydration via the change in the solubility (the perovskite solution). Moreover, to confirm the MT introduction effect, the optoelectronic properties of a perovskite photodetector were evaluated under each H2O content in the solutions. The results confirmed that photoresponse is efficiently enhanced by MT introduction. In addition, MT-based perovskite photodetector devices exhibited the stable driving characteristic and reliability despite the moisture content. This phenomenon was verified through X-ray photoelectron spectroscopy and Raman analysis of the perovskites, whose results prove the proposed perovskite degradation mechanism.