The invention discloses a nano-photocatalyst. The nano-photocatalyst is prepared from, by weight, 30-35 parts of nanometer titanium dioxide, 5-10 parts of nano microcrystalline cellulose colloid (nano microcrystalline cellulose accounts for 2-3.5% by weight), 2-6 parts of adhesive, 1-3 parts of a titanate silane coupling agent, 1-3 parts of anion surfactant, 5-10 parts of dispersant and 80-85 parts of deionized water, wherein the nanometer titanium dioxide is porous anatase type nanometer titanium dioxide, the particle size of the nanometer titanium dioxide is 3-5 nm, and the nano microcrystalline cellulose in the nano microcrystalline cellulose colloid is of a net structure. The nano-photocatalyst and the preparation method thereof are environmentally friendly and free of pollution, the particle size of the porous anatase type nanometer titanium dioxide is 3-5 nm, the specific surface area is large, the nano microcrystalline cellulose of the net structure connects nanometer titanium dioxide to the surface, the distribution range of the nanometer titanium dioxide is expanded, the contact area between the nanometer titanium dioxide and a substrate is increased, and the purifying effect is improved. By means of the nano-photocatalyst, harmful gas degradation is conducted on newly-built shopping malls, large-scale office buildings, newly-decorated dwelling houses and the like, and harmful substances are converted into active substances such as [O] and [HO].