Adsorption of acidified multiwalled carbon nanotubes (MWCNTs) to heavy metal using Pb(Ⅱ) as a model was investigated and characterized by many techniques. The main adsorption mechanism of acidified MWCNTs to Pb(Ⅱ) is proposed on the basis of adequate analysis. The results show that the oxygenous functional groups can be formed on MWCNTs after MWCNTs were treated by concentrated nitric acid. The oxygenous functional groups play an important role in Pb(Ⅱ) adsorption to form chemical complex adsorption, which accounts for 75.3% of all the Pb(Ⅱ) adsorption capacity. The Pb(Ⅱ) in the form of PbO, Pb(OH)2, and PbCO3 adsorbed on the surface of the acidified MWCNTs is only 3.4% of the total Pb(Ⅱ) adsorption capacity. The Pb(Ⅱ) species adsorbed on acidified MWCNTs mainly aggregate on the ends and at the defects sites on the acidified MWCNTs.