Adsorption behavior is rarely investigated as a heavy metal solidification mechanism in alkali-activated municipal solid waste incineration (MSWI) fly ash compacts. This study simulates the adsorption behavior and solidification mechanism of Pb(II) on synthetic C-S-H gel with low and high Ca/Si ratios in high alkaline internal pore solution environments of alkali-activated MSWI fly ash compacts. The study examines the fraction of Pb(OH)3- in a Pb(II) solution (100 mg/L) at pH 11.8 and 12.4, with percentages of 85.9–90.6 % and 93.4–95.1 %, respectively. pH values are identified as the key parameter affecting the adsorption capacity of C-S-H for Pb(II). The equilibrium adsorption capacities (Qe) values of Pb(OH)3- (pH = 11.8) for C-S-H with Ca/Si ratios 1.20 and 0.60 are approximately 104 mg/g and 110 mg/g. At pH 12.4, the C-S-H gel with a Ca/Si ratio = 1.20 performs better in terms of adsorption. The adsorption of Pb(OH)3- by C-S-H is predominantly a chemisorption process at pH 11.8. Based on the ΔG0 data, the adsorption process is determined to be spontaneous. The 1.16 eV LUMO and HOMO difference for Si2O(OH)6 + Pb(OH)3-, compared with the 3.92 eV difference for Si3O2(OH)8 + Pb(OH)3-, indicates a lower difficulty of electron transition in the reaction between Si2O(OH)6 and Pb(OH)3-. XPS and FTIR results confirm the dehydration and combination of Pb(OH)3- and Si-OH in a silicate tetrahedron (Q1 or Q2) on the C-S-H surface to create Si-O-Pb. Partial Ca(II) in C-S-H gel can be replaced or released during the Pb(II) adsorption process. These findings hold significant theoretical importance for the adsorption and solidification mechanism of Pb(II) in alkali-activated MSWI fly ash specimens.