Abstract: The middlings product produced by Tunlan Plant can hardly meet the requirement of power plant because of its high ash and low calorific value. Based on analysis of the structural features of the 3GDMC1200 /850 A gravity- fed 3- product heavy medium cyclone as well as related experimental data,technical remoulding work is made on the 2nd- stage of the cyclone under the prerequisite that both efficiency and separating precision of the cyclone’s 1st- stage are not affected. Application of the renovated cyclone makes it possible for the 2nd- stage to realize real- time adjustment and control,while the ash of the middlings product can be maintained stably in range of 33%- 35%,with a calorific value of about 18. 22 MJ/kg. This not only ensures the production of up- to- standard middlings product,but also provides a good reference for other plants to make technical reform of similar types of cyclones in the future.