The invention discloses a normal temperature preparation method of a hydrated molybdenum trioxide photocatalyst, which is mainly characterized in that sodium molybdate and nitric acid are adopted as raw materials to react at an temperature of 20 to 30 DEG C to prepare hydrated molybdenum oxide nano-rod powder on a large scale. Under the irradiation of visible light, the prepared hydrated molybdenum oxide nano-rod powder achieves very good treatment effects when being used as the catalyst to treat simulated printing and dyeing wastewater of methylene blue, rhodamine B, methyl orange, and the like, and all decoloration rates exceed 98 percent.