The cocombustion of coal with biomass for energy generation has emerged as a method with tremendous potential in the protection of the environment and the upgrade of the energy structure. The transformation behavior of toxic elements (TEs) during the combustion of coal with biomass does not follow a simple additivity rule. The laboratory-scale fluidized bed reactor is utilized to elucidate the effect of biomass types (corn stalk and sawdust) on the transformation behavior of the TEs during cocombustion. The sequential chemical extraction and thermodynamic equilibrium calculation are applied to investigate the transformation mechanism of the TEs. Results show that the redistribution characteristics of the TEs are feedstock dependent. The gaseous emissions of most TEs are reduced obviously during the cocombustion of coal with biomass, which is attributed to the interactions of the TEs with alkali (earth) metals. As, Cd, Sb, and Se are favored to react with Ca and Fe, while Pb, Cu, and Zn have positive proportions to alkali metals (Na and K). The association of the TEs, thermal stability of the host matrix, and interaction with ash composition play significant roles in the transformation behavior of the TEs during thermal treatment. The cocombustion residues are thought to be a medium risk to the ecological environment due to the elevated liberation of the TEs. Stringent management and countermeasures should be executed during the utilization of these cocombustion residues.