The method of pipe jacking has been widely used in underground engineering. However, the research regarding the dynamics and mechanical responses of soil during the excavation process of the innovative combined pipe-jacking machine with a large section is still insufficient. A full-scale model of the combined pipe-jacking machine with the largest section in the world thus far was established. The excavation and discharge process of the machine was modeled using DEM (discrete element method), and the effects of discharge rates on the dynamics and responses of soil were analyzed. The results show that the optimal ratio of advance-discharge rate is approximately 0.6 cm/rev. The fluidity of soil is primarily influenced by the two cutterheads located at the bottom of both half sections. In the case of excess-discharge, the soil pressure of the stratum cumulatively increases, and the ground surface is prone to rising. In contrast, in the case of excess-discharge, the soil pressure is less than the static pressure, and a local failure of the covering stratum is formed, and then develops into a complete failure as the machine advances.