Due to weak affinity of the adsorbents for As(III), the arsenic removal, especially in the form of H3AsO3, remains to be highly challenging. Herein, the MgAl-layered double hydroxide (MgAl-LDH) and the MgFeAl-LDH are prepared successfully by coprecipitation method. Under full spectrum illumination, the MgAl-LDH shows As(III) removal from 20 mg/L to 2.19 mg/L, while the MgFeAl-LDH is able to reduce As(III) from 20 mg/L to 0.025 mg/L, which is two orders of magnitude lower than that by MgAl-LDH and significantly below the groundwater limit of 0.05 mg·L−1 of As(III). Moreover, the removal capacity of MgFeAl-LDH is 2.2 times (101.6 mg g−1) that of MgAl-LDH (45.7 mg g−1). Note that the MgFeAl-LDH can reach equilibrium in 1 h and the kinetic curve is in good agreement with the pseudo-second-order model. XPS and DFT reveal that the presence of Fe3+ in MgFeAl-LDH increases the electron density of Al3+ via electron transfer from Fe3+ to Al3+, reducing the adsorption energy of As on Al3+ sites and efficiently activating Al3+, while the adsorption sites expand from single Mg2+ sites in MgAl-LDH to Mg2+, Al3+ and Fe3+ sites in MgFeAl-LDH. Furthermore, EXAFS indicates that the incorporation of Fe3+ in MgFeAl-LDH results in enhanced photo-oxidation of H3AsO3 to HAsO42− and extends the light absorption by generating intermediate energy levels, thereby improving the As removal capacity.