With the spread of phytoremediation, research on the behavior of heavy metals (HMs), such as Cd, during the heat treatment of contaminated biomass is necessary. The thermodynamic equilibrium calculation was used to forecast the form of Cd during the combustion of ryegrass. When co-existing with Pb and Zn, Cd mainly existed as CdO for the prior combination between trace Cl and numerous K/Mg. The combustion experiment with mineral additives was conducted in the horizontal tube furnace, and the sub-step leaching procedure was also used to evaluate the leaching toxicity of products. During combustion at 900 degrees C without additives, about 93.7% Cd transferred to the atmosphere. Kaolin, montmorillonite, and gamma-Al2O3 provide active Si or Al to restrain the volatilization of Cd and reduce the concentration of Cd in the leaching solution via similar mechanisms, while montmorillonite is more efficient on Cd retention than kaolin over 900 degrees C. According to the X-ray diffraction analysis, reactions between the K and Si/Al components may form K-Si-Al compounds, such as KAlSiO4, and restrain the retention of Cd directly. Although the addition of CaO promotes the volatilization of Cd and other HMs, it can restrain the complexation of cadmium cations and reduce the leaching toxicity greatly by raising the pH value.