Biochar has considerable sorption efficiency for organic pollutants; however, the effect of physicochemical characteristics and their alteration by environmental conditions on the sorption mechanism is still unknown. We investigated the pH-dependent sorption of simazine on Miscanthus biochar produced at two pyrolysis temperatures (400 and 700 °C; hereafter B-400 and B-700) under two different electrolytes and interpreted the sorption mechanism. The surface charge density (SCD) decreased more in B-400 than in B-700 at higher pH due to more deprotonation of acidic functional groups (AFGs), but greater decreases were observed in B-700 than in B-400 from pH 2 to pH 3 as a result of alkali salts deposition. The decrease in KF with increasing pH showed that simazine sorption decreased as van der Waals forces because the surface of biochar carried a greater negative SCD, which repulses simazine molecules due to the enhanced deprotonation of AFGs. At a given pH, KF was lower in CaCl2 than in NaCl due to the formation of larger metal-biochar complexes, resulting in enhanced blocking of pores available for simazine sorption. We believe that knowledge of the pH-dependence of SCD and accessibility of biochar pores could help better interpret the behavior of simazine-like pollutants in soil and aquatic environments.