Lignin biochar was acid washed and then treated with ultrasound at different time durations to improve its sorption of CO₂. Raman, XRD, FTIR spectrum and thermostability test confirmed that this post-treatment method cleaned the surface of the pristine biochar and enlarged its porosity by opening pores blocked by ash and organic components. As a result, specific surface areas and micropore volumes of post-treated biochars increased 7.3–8.6 and 8.2–8.8 times, respectively, while their ash contents decreased from 76.31% to 6.12%-13.63%. More importantly, CO₂ uptake of the post-treated biochar reached 178.75 mg/g, which could be ascribed to developed micropore structure. Adsorption kinetic and isotherm analyses showed that CO₂ was adsorbed onto the post-treated biochar mainly through multilayer physical adsorption. The high adsorption capacity, terrific reusability (99.46% after 10 cycles) and excellent selectivity of the post-treated biochar suggest that acid washing assisted with ultrasonic treatment is a promising modification technique for biochar, promoting its engineering application in CO₂ capture.