In this study, sludge-based pyrolysis bio-char was obtained and activated with chemical activators (KOH and CH3COOK) and a physical activator (CO2). The adsorption of the heavy metal Pb(II) from aqueous solution onto the sludge-based biochars was explored. Results showed that the specific surface area was increased significantly by more than 10 times via KOH activation, followed by CH3COOK activation and CO2 activation. The activation of CO2 or CH3COOK facilitated the introduction of the oxygen-containing functional groups, which are much better than that of KOH. Better Pb(II) adsorption capacity (57.48 mg/g) was obtained by KOH activation than other studied biochars, since physical adsorption was dominant in the adsorption process accompanied by chemical adsorption. During chemical adsorption the adsorption site was related to C=O, and Pb adsorbed on the surface of sludge-based biochar was mainly in the form of PbCO3. Both Langmuir and Freundlich isothermal adsorption models can describe the process of Pb(II) adsorption on sludge-based biochar.