Abstract: The effective prevention and control of invasive alien species remains a technical challenge, and the resource utilization of these plants offers a new approach for managing invasive plants. This study used stems (EBC) and a mixture of stems and leaves (ECBC) of the invasive alien plant Erigeron canadensis L. to prepare biochar under limited oxygen conditions at 300 ℃, 500 ℃, and 700 ℃, followed by modification with sodium hydroxide (alkali) and citric acid (acid). The differences in physicochemical properties—including yield, pH, specific surface area, functional groups, mineral composition, and morphology—of biochar produced from different plant parts, pyrolysis temperatures, and modification conditions were systematically compared. The adsorption performance for As(Ⅲ) and Cd(Ⅱ) in aqueous solution was also investigated. The results showed that as the pyrolysis temperature increased, the biochar yield decreased, while the pH and specific surface area increased significantly. Specifically, acid-modified ECBC prepared at 700 ℃ (ECBC700) exhibited the largest specific surface area of 108.13 m²/g, which was 178% higher than that of the unmodified biochar. The abundance of oxygen-containing functional groups in the biochar decreased with increasing temperature but increased after modification. X-ray diffraction analysis confirmed the presence of abundant minerals in the biochar, with ECBC showing a greater diversity of mineral types. Due to its richer mineral elements and more developed pore structure, ECBC exhibited significantly higher specific surface area, more diverse mineral types, and superior adsorption capacity for both Cd(Ⅱ) and As(Ⅲ) compared to the stem-only biochar (EBC). In a 20 mg/L heavy metal solution, ECBC700 achieved a removal rate of 99.1% for Cd(Ⅱ), significantly higher than that for As(Ⅲ) (14.4%). The biochar exhibited high adsorption performance for As(Ⅲ) in lower pH solutions and for Cd(Ⅱ) in higher pH solutions. The pseudo-second-order kinetic model (R²>0.99) indicated that the adsorption of As(Ⅲ) and Cd(Ⅱ) onto Erigeron canadensis L. biochar was dominated by chemical adsorption. The unmodified biochar ECBC700 showed the highest adsorption capacity for Cd(Ⅱ), while the acid-modified ECBC700 exhibited the highest adsorption capacity for As(Ⅲ). This study reveals that Erigeron canadensis L. biochar exhibits favorable adsorption performance for As(Ⅲ) and Cd(Ⅱ) in aqueous solutions through synergistic mechanisms including physical adsorption, surface complexation, precipitation, ion exchange, and electrostatic attraction. It provides a feasible path for the resource utilization of invasive alien plants.