Abstract A laboratory-scale circulating fluidized bed combustion experiment was performed to investigate the retention mechanism and transformation behavior of trace elements (TEs) during co-combustion of coal gangue (CG) with soybean stalk (SS). Chemical sequential extraction was conducted to determine the associations of the TEs. Results show that the distribution and transformation behavior of the selected TEs during co-combustion are variable, which is attributed to the different physico-chemical properties, associations of the TEs and ash components. The elevated retention of gaseous TEs during co-combustion may be explained by the surface-reactions with active minerals, chemisorption, and the interactions with ash components and elements. The gaseous phase, exchangeable and residual fractions are increased with the decomposition of the Fe-Mn oxides fraction in the oxidizing atmosphere. The combustion ash may pose a medium environmental risk if no countermeasure is adopted.