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Nitrate is difficult to form a precipitate due to its strong electronegativity, water solubility, making it difficult to be removed by rapid adsorption. Nitrate adsorption can be enhanced by increasing the positive charge of the adsorbent surface through chemical modification. The montmorillonite was modified with poly dimethyl diallyl ammonium chloride (PDADMAC), and PDADMAC was successfully loaded onto the surface of montmorillonite, and the adsorption mechanism was compared and analyzed by characterizing montmorillonite(Mt) before and after modification. The results show that: the content of quaternary amine nitrogen in modified montmorillonite (PDM-Mt) increases, and the surface potential increases significantly. The NO3--N adsorption capacity of PDM-Mt is 3.07 mg/g at pH=7, which is 86% higher than that before modification. The adsorption process is in accordance with the quasi-second-order kinetic model as well as the Langmuir adsorption isothermal model, that means it is mainly single-molecule layer chemisorption. The NO3--N adsorption process reaches equilibrium within 4 h, and the adsorption effect is obvious in the range of pH 5-7. After repeated 5 times, the adsorption capacity decreased by 21%, showing high recyclability. PDADMAC has the potential as a modifier to promote nitrate adsorption. [ABSTRACT FROM AUTHOR]