The adsorption experiment of As(Ⅲ) in water was studied, and the ultrasonic-assisted ferric salts modified phosphorus slag for adsorbent was used. The effects of the variety and concentration of ferric salts, ultrasonic time in the process of modification on the adsorbing capacity for As(Ⅲ) by using modified phosphorous slag have been investigated. The optimal preparation conditions are as follows:0.8mol/L of FeCl₃, and ultrasonic time of 15min. The methods FT-IR, SEM and XPS were used to analyze the difference of the modified and un-modified phosphorous slag. After modification, Fe³⁺ can be a very good load in the surface of phosphorus slag and the removal rate of As(Ⅲ) in wastewater was improved to 98.9%, compared with the unmodified, increased by 38%. The kinetics of adsorption process fitted in with pseudo second order reaction, the fitting correlation coefficient (R²) is above 0.96; The adsorption isotherm could also satisfy both isotherm model of Langmuir and Freundlich simultaneously, the fitting correlation coefficient (R²) is above 0.90. In comparison, the Langmuir isotherm model has a better description of the adsorption process.