Coal chemical gasification process produces a large number of gasification fine slag，which does not meet the national and industrial standards of building mixed raw materials due to its high carbon content and large burning loss. Because of the lack of effective large-scale consumption mode，gasification fine slag with huge output has become an important problem restricting the sustainable development of coal chemical enterprises at the present stage. Through the mechanism study of the preheating and decarbonization process of a constant temperature preheating and decarbonization device for low volatile and low calorific value fuel，the constant temperature thermogravimetric test is carried out by using the thermogravimetric experimental platform to simulate the internal combustion process of the constant temperature preheating and decarbonization device for low volatile and low calorific value fuel，so as to compare and analyze the combustion characteristics of particle size graded gasification fine slag under different preheating temperatures and different combustion atmospheres. It is found that the gasification fine slag samples begins to oxidize rapidly when oxygen is introduced，and the burnout time is between 6.6-9.4 min at 900 ℃ and 10% O2，and 3.7-5.6 min at 900 ℃ and 21% O2. Under the condition of controlling the NOx value within the specified range，the oxygen concentration in the combustion zone can be appropriately increased to shorten the burnout time. With the increase of preheating temperature，the average weight loss rate of gasified fine slag samples with the same particle size grading increases regularly，while the burnout time decreases regularly. The increase of preheating temperature can improve the burnout characteristics of gasified fine slag，and the combustion zone temperature can be appropriately raised to achieve faster burnout under the safe operation of the equipment. Under different combustion atmosphere and preheating temperature，with the increase of particle size of gasification fine slag，the weight loss increases，the burnout time increases regularly，and the average weight loss rate decreases regularly. Therefore，the preheating and decarburization device needs to reasonably adjust the residence time of materials according to the particle size of materials in order to achieve full burnout.