In order to improve the adsorption capacity of attapulgite(ATP)to As(Ⅲ)in water, lanthanum chloride heptahydrate and ATP were used to prepare lanthanum modified attapulgite(La-ATP)by impregnation method. Through response surface optimization test, the optimal modification conditions were explored, and the isothermal thermodynamics and adsorption kinetics of As(Ⅲ)in water were tested. The results show that the optimal concentration of lanthanum chloride is 0.6 mol/L, the optimal dosage of adsorbent is 0.3 g, and the optimal reaction pH value is 6. Through the response surface optimization test, the optimal modification conditions are as follows: the concentration of lanthanum chloride is 0.61 mol/L, pH value is 6.8 and the dosage of adsorbent is 0.38 g. At this time, the adsorption capacity of La-ATP to As(Ⅲ)is 166 μg/g. The adsorption isotherm is in accordance with Freundlich equation, and the adsorption process is spontaneous and endothermic. The quasi second-order model can more accurately describe the adsorption kinetic process, indicating that the chemical adsorption plays a major role in the adsorption process.