The coexistence of cadmium (Cd(II)) and arsenate (As(V)) pollution has long been an environmental problem. Biochar, a porous carbonaceous material with tunable functionality, has been used for the remediation of contaminated soils. However, it is still challenging for the dynamic quantification and mechanistic understanding of the simultaneous sequestration of multi-metals in biochar-engineered environment, especially in the presence of anions. In this study, ferrihydrite was coprecipitated with biochar to investigate how ferrihydrite-biochar composite affects the fate of heavy metals, especially in the coexistence of Cd(II) and As(V). In the solution system containing both Cd(II) and As(V), the maximum adsorption capacities of ferrihydrite-biochar composite for Cd(II) and As(V) reached 82.03 µmol/g and 531.53 µmol/g, respectively, much higher than those of the pure biochar (26.90 µmol/g for Cd(II), and 40.24 µmol/g for As(V)) and ferrihydrite (42.26 µmol/g for Cd(II), and 248.25 µmol/g for As(V)). Cd(II) adsorption increased in the presence of As(V), possibly due to the changes in composite surface charge in the presence of As(V), and the increased dispersion of ferrihydrite by biochar. Further microscopic and mechanistic results showed that Cd(II) complexed with both biochar and ferrihydrite, while As(V) was mainly complexed by ferrihydrite in the Cd(II) and As(V) coexistence system. Ferrihydrite posed vital importance for the co-adsorption of Cd(II) and As(V). The different distribution patterns revealed by this study help to a deeper understanding of the behaviors of cations and anions in the natural environment.