A novel adsorbent, lanthanum hydroxide-doped activated carbon fiber (ACF-LaOH), was prepared by the ultrasound-assisted chemical precipitation method for phosphate removal from waste water. Ligands exchange, electrostatic interactions and Lewis acid–base interaction are the three main mechanisms for phosphate adsorption. The ligands exchange, electrostatic interactions would become weaker and the Lewis acid–base interaction be strengthened with the increase of pH values giving rise to the decrease of adsorption amount. Display omitted ► The preparation and performance of ACF-LaOH for phosphate removal were investigated. ► The adsorption mechanisms were investigated by SEM, FT-IR and pH analysis. ► Ligands exchange, electrostatic interactions and Lewis acid–base interaction are the 3 mechanisms. ► Ligands exchange, electrostatic interactions are weaker with the increase of pH values. A novel adsorbent, lanthanum hydroxide-doped activated carbon fiber (ACF-LaOH), has been prepared by the ultrasound-assisted chemical precipitation method for phosphate removal from waste water. Based on the single-factor method, response surface methodology (RSM) by using a Box–Behnken design (BBD) was applied to assess the mutual interactions and effects between the three factors and the optimized preparation conditions (concentration of La3+ of 0.11mol/L, ultrasonic power at 206W and ultrasonic time of 7.3min). Adsorption kinetics and isothermal adsorption studies showed that the pseudo second-order model and the Langmuir isotherm fitted the experimental data quite well, indicating that the adsorption process was mainly through chemical interactions. At last whereas most importantly, the phosphate adsorption mechanism was investigated by analyzing the scanning electron microscopy (SEM), fourier transform infrared (FT-IR) spectroscopy and the relationship between the adsorption amount and the pH of phosphate solution. The results demonstrated that ligands exchange, electrostatic interactions and Lewis acid–base interaction were the three main mechanisms for phosphate adsorption. The ligands exchange and electrostatic interactions became weaker, but the Lewis acid–base interaction was strengthened with the increase of pH values giving rise to the decrease of adsorption amount.