Abstract: With the increasing efforts of oil and gas exploration and development and the increasing drilling depth, the downhole vibration of ultra-deep drilling string is very complex, and the stress state changes significantly with time. In order to ensure the safety of ultra-deep drilling string, the dynamic fatigue failure characteristics of ultra-thin drilling string constrained by space deflection wellbore are studied, and the structure and working parameters of drilling string are optimized. Based on actual well trajectory, considering the collision characteristics of the drill string with the borehole wall, the dynamic characteristics of the drill string are obtained through finite element simulation analysis. And the fatigue strength of the drill string in the asymmetric cyclic stress state is studied according to the accumulated fatigue damage theory. Combined with the actual situation in the field, the dangerous cross-section of the drill string of ultra-deep wells is given, and the variation law of fatigue failure of the drill string with rotation speed, WOB and stabilizer position is studied. The researches show that WOB and high rotational speed have greater effect on the fatigue strength of the drill string, and low level rotational speed has a lesser effect on the fatigue strength of the drill string. After installing the stabilizer, the probability of BHA fatigue failure can be greatly reduced, and its placed position has a significant effect on the fatigue strength of the drill string. The research results provide a theoretical basis for the optimization of drill string structure parameters and drilling parameters in ultra-deep wells.