Abstract: Disclosed is a composition for inhibiting nitrate decomposition and its preparation method, which belongs to a field of photocatalytic technology, comprising: weighing titanium dioxide and pure phase metal carbonate or pure phase metal bicarbonate proportionally; adding the weighed pure phase metal carbonate or the pure phase metal bicarbonate to titanium dioxide for grinding to obtain a metal carbonate/bicarbonate-containing mixture. The method of inhibiting nitrate decomposition using the metal carbonate/bicarbonate of the present disclosure has a significant ability to inhibit nitrate decomposition, and the experimental results show that the method of inhibiting nitrate decomposition using the metal carbonate/bicarbonate can effectively inhibit the decomposition of the nitrate under irradiation for a long time.