The stability of the cavern in the process of underground coal gasification is an important issue for ensuring gasification production. Based on the formation physical properties, dimensions, thermodynamic parameters and rock mechanics parameters of the coal formation in the Shaerhu Sag of the Tuha-Santanghu Basin, the geological-cavity model is established by the discrete element method, and the different pressures of different cavities are studied. Under the conditions of creep, the distribution of the plastic zone of the cavity is obtained, and the subsidence of the ground surface is obtained by calculating the long-term creep behavior of the cavity. The conclusion shows that the plastic zone tends to be distributed on the inner wall of the cavity, and on the whole it is dominated by shear failure. There is no large-scale outward expansion within 1 m of the cavity wall. As for a cavity with a radius of 5 mat 1000 m underground, the subsidence to the surface caused by the cavity creep itself is relatively limited, and the main source of surface subsidence is the creep of the coal formation itself. This research provides guidance for the evaluation of the stability of the cavity during the operation and abandonment of underground coal gasification.