The radioactive uranyl and thorium ions discharged into freshwater have been the imperative environmental issue arousing great concern. Biochar, owing to its low cost, operational ease, and high efficiency, is touted as the promising sorbent for the U(VI) and Th(IV) removal in aqueous solutions. This study employed several characterization techniques and batch experiments to investigate mechanisms of U(VI) and Th(IV) adsorption on biochar. The kinetic results exhibited the fitness of the pseudo-second-order model. Both U(VI) and Th(IV) removals were influenced by pH, while only the ionic strength effects on U(VI) adsorption were observed. The adsorption of either the U(VI) or the Th(IV) fitted to the Langmuir and Freundlich model well. Natures such as endothermicity, increasing randomness, feasibility, spontaneity, and temperature favor of the adsorption reaction could be found in U(VI) and Th(IV) adsorption (U: ΔH0 = 36.28, ΔS0 = 108.84 and ΔG0 < 0; Th: ΔH0 = 63.42, ΔS0 = 134.12 and ΔG0 < 0). Our results highlighted the importance of biochar as the potentially green adsorbent and provided a practical strategy for radioactive contaminant remediation.