The excellent optoelectronic properties of metal halide perovskites (MHPs) have been employed in various photocatalytic applications, but their poor water stability is considered as the main bottleneck for further development. Herein, we protect the light-absorbing CsPbBr3 MHP with a NiOx and TiO2 hole and electron extracting layer. This planar NiOx/CsPbBr3/TiO2 architecture can easily be fabricated through solution-processing. When applied to selective photocatalytic oxidation of benzyl alcohol, this system presents a 7-fold enhancement of photoactivity and an improved stability for over 90 h compared to CsPbBr3 counterpart. Interestingly, we find that trace amounts of water improve photoactivity. Through experimental and theoretical analyses, this improvement could be attributed to water-induced structural reorganization of MHP, leading to improved crystal quality and decreased effective masses of charge carriers. This work indicates planar heterojunction helps improve the photoactivity and stability of MHP photocatalyst, and our findings provide insights into the effect of water on MHPs.