Cuprous oxide (Cu2O) microcrystal is an excellent material for absorbing visible light and photo‐catalyzing various water pollutants. The photocatalytic performance of Cu2O crystal mainly depends on its morphology and exposure surface. In this study, Cu2O crystals have been successfully fabricated at room temperature. Morphology and the proportion of exposed crystal faces are regulated by a simple method with hydrazine hydrate as the reducing agent. The experimental results unveiled that high dosage of hydrazine hydrate tends to produce crystals with spherical and low (111) plane ratios, while low dosage of hydrazine hydrate provides an excellent condition for the formation of octahedral microcrystal with high proportion of (111) planes. The photocatalytic degradation efficiency of methylene blue (MB) in visible light increased with the proportion of (111) planes. Analysis shows that the better crystallinity and conductivity of the octahedron structure improved the utilization rate of photogenerated carriers, while the unsaturated Cu on the (111) planes as the active site promotes the decomposition of the pollutants.