Available information about the passivation effect on heavy metals (HMs) through adsorption and humification during zeolite-amended composting remains limited. Thus, this study explored the dynamic changes in HM-fractions (Zn, Cu, Cd, Cr and Pb) during aerobic composting added with different grain-size zeolite (Fine zeolite, <0.1mm, ZF; Coarse zeolite: 3-5mm, ZC). Compared to the control (without zeolite, CK) and ZF treatments, ZC treatment got the highest temperature in the thermophilic phase, and significantly reduced the bioavailability factor (BF) of HMs, especially for Cu (45.13%), Cd (16.11%) and Pb (25.49%). Redundancy analysis (RDA) and structural equation models (SEMs) indicated that zeolite accelerated the passivation effect on Cd and Pb through regulating the electrical conductivity (EC) as a result of surface adsorption, and on Cu by influencing total carbon (TC) under the function of humification. These results increase our understanding of the passivation mechanisms of HMs during aerobic composting.