Geopolymers have been widely applied to pretreat Pb-containing solid wastes due to low carbon footprint and superior stabilization/solidification (S/S) performance. However, the wastes may be exposed to rainwater or water flow during long-term landfill, leading to the decalcification of internal geopolymer structure. Here, the influence of decalcification on the S/S performance of Pb-containing geopolymers was investigated through combined inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), differential Thermogravimetry (DTG), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), and mercury intrusion porosimeter (MIP) tests. Results show that decalcification can increase the cumulative leaching concentration of Pb and degrade the S/S performance of geopolymers due to the release of [Pb(OH)4]2- ions from the end-of-chain silicates of calcium aluminosilicate hydrates and the dissolution of Pb(OH)2 precipitates. A leaching model further indicates that surface wash off, i.e., the dissolution of Pb(OH)2 precipitates from the surface, is the main factor controlling degradation. In addition, for evaluating potential hazards of Pb leakage, electrochemical impedance spectroscopy (EIS) test was proposed to precisely predict the cumulative leaching concentration of Pb. Overall, this study is an innovative attempt to reveal the changes of the S/S characteristics of geopolymers under decalcification and track the leaching behaviors of Pb in landfill sites.