The ultra-deep diaphragm wall of the soft soil layer has a large depth of base slot, low soil strength, poor stability, long slotting time, difficult to control the stability of the slot wall, and high risk of instability in the construction process. Based on the GX12 ultra-deep diaphragm wall working well in the "Hanjiang-rongjiang-lianjiang river system connectivity follow-up optimization project", the verified numerical simulation method is used to study the influence law of groundwater depth, mud parameters and sand permeability on the stability of the ultra-deep diaphragm wall in the deep soft soil and sandy composite strata. The results show that: 1) The horizontal displacement and surface settlement of the slot wall decrease with the increase of the depth of the original groundwater level, and gradually tend to be stable. When the original water level is -1.28 and -10.28 m, the horizontal displacement of the slot wall is 33 mm and 18 mm, respectively, which is reduced by about 46%, and the surface settlement is reduced by about 57%; 2) The horizontal displacement of the slot wall and the surface settlement decrease with the increase of the slurry gravity or the height of the slurry level. When the slurry gravity is less than 12.5kN/m³ , it is difficult to meet the stability requirements of the slot wall, and when the slurry increases from 12.5kN/m³ to 13.0kN/m³ , the horizontal displacement of the slot wall decreases by about 68% , and the surface settlement decreases by about 55% . When the slurry level height decreases from 2.5m to 1.5m , the horizontal displacement of the slot wall increases by 300% and the surface settlement increases by 304.5% . It is recommended that the slurry gravity should not be less than 12.5kN/m³ , and the height of the slurry level should not be less than 2.0m;3) . Under the 4 working conditions of silt, fine sand, medium sand and coarse sand in the confined aquifer, the deformation of silt is the smallest, the fine sand is the largest, the horizontal displacement difference is about 50% , and the surface settlement difference is about 53% .