Abstract: The immobilization/stabilization effect and mechanism research of carbon-based amendments on Cd and Cu in contaminated soils have always been the key and pivotal problem in this field. In this study, Cd and Cu co-contaminated paddy soil was used as the test soil, and unmodified biochar (BC), iron-modified biochar (FeBC), and iron-modified activated carbon (FeC) served as amendments. The immobilization effects and aging mechanisms of these amendments on Cd and Cu in the soil were investigated by high-temperature experiments to simulate the natural aging processes. The results show that all of the amendments could immobile soil Cd and Cu. With the amendments, the soil Cd contents in exchangeable and carbonate bound fractions (F1) were reduced by 3.0%, 9.4%, and 8.8% respectively, and soil Cu contents in the F1 were reduced by 2.2%, 4.4%, and 3.8% respectively. Moreover, under the action of amendments, the contents of amorphous iron oxide bound fractions (C-Feo) of soil colloid Cd were increased by 3.5%, 4.9%, and 4.5%, respectively. As for the soil colloid Cu, its contents grew by 14.5%, 18.8%, and 18.1%, accordingly, which emerged as the primary reason and path leading to short-term reduced bioavailability of these heavy metals. Furthermore, the results of the high-temperature aging experiment show the transformation from the C-Feo to the crystalline iron oxide bound fractions (C-Fec) and the residual fractions (C-Fr) of soil colloids Cd and Cu, indicating the three amendments can exert long-term immobilization effect on soil Cd and Cu.