The behavior of lead (Pb) during thermal treatment of municipal solid waste (MSW) is a serious environmental concern. The migration of Pb during pilot-scale thermal treatment of MSW with controlled air excess ratio (ER) was studied with a focus on Pb contents and isotope ratios analysis. Different ERs showed different Pb distribution behaviors in fly ash (FA) from MSW incineration, owing to the change of Pb migration from different MSW components. Although the Pb contents in FA under the oxidizing condition increased significantly with the increase of ER (almost 100% from ER = 1.0 to ER = 1.3), the major sources (i.e., papers and plastics) contributing Pb to FA were similar based on the Pb isotope identification. This suggested that the migration of Pb from these MSW components was promoted in a high oxygen environment. In contrast, the Pb contents in FA under the low oxygen condition (ER = 0.3-0.5) were similar, and rubbers became the major source of Pb in FA in the low oxygen environment instead of papers. In the low oxygen environment the migration of Pb in rubbers and papers was promoted and inhibited, respectively, as indicated by isotopic analysis combined with micro-X-ray fluorescence and diffraction analyses.