The invention discloses a method for preparing a photocatalyst by using a nanocrystalline cellulose template and belongs to the field of photocatalyst preparation. The method for preparing the photocatalyst by using the nanocrystalline cellulose template comprises the steps that nanocrystalline cellulose colloid, zinc acetate, silver nitrate and sodium borohydride are added to absolute ethyl alcohol, and an NCC/ZnO/Ag compound is obtained. The compound is subjected to centrifugal washing, vacuum freeze-drying and high-temperature calcination, and the ZnO/Ag photocatalyst with high catalysis capacity is prepared. According to the prepared photocatalyst, with the help of the high specific surface area of nanocrystalline cellulose, the ZnO/Ag photocatalyst is endowed with high specific surface area and reaction activity. It is proved through performance tests that the photocatalytic degradation rate of the ZnO/Ag photocatalyst prepared through the nanocrystalline cellulose template to 20mg/L of methylene blue under illumination of visible light within 2 h reaches 85-95%.