Fluoride (F) and arsenic (As) are two typical and harmful elements that are found in high concentrations in geothermal water in Tibet. In the present work, yak dung, an abundant source of biomass energy in Tibet, was made into biochars (BC1, BC2 and BC3) by pyrolysis under different conditions, and the better biochar was modified by FeCl2 (Fe-BC3). The adsorption conditions were optimized to adsorb F and As in geothermal water. The results showed that BC3 can remove 90% F- and 20% As(V), which is the best effect of the three initial biochars. Fe-BC3 could remove 94% F- and 99.45% As(V) under the same conditions as BC3, which was an adsorbent dosage 10 g/L, pH 5-6 and temperature of 25 °C. It was also demonstrated that the removal rate did not decrease at 80 °C. A quasi-second-order kinetic model best described the adsorption behavior of ions on the surface of the biochar. The maximum adsorption capacity of F- and As(V) on Fe-BC3 was 3.928 mg/g and 2.926 mg/g, respectively. The features of Fe-BC3 were characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Brunauer-Emmett-Teller (BET), Energy Dispersive Spectrometer (EDS), and Scanning Electron Microscopy (SEM) to understand the adsorption process.