In this study, the cause of the fan shaft failure and fatigue failure of S45C steel was analyzed. The fatigue cycle of the broken fan shaft was estimated to be approximately 3.21×106, and the bending stress was estimated through the fracture analysis and ANSYS mechanical structural analysis. FEM analysis of the stress change and displacement of the shaft was performed assuming self-aligning roller bearings and deep groove ball bearings. When self-aligning roller bearings were applied, the deformation increased by approximately 60 %, but the concentrated stress decreased, making the shaft more stable under the load. Additionally, by improving the shaft step radius from 0.5R to 2.5R, a stress reduction effect of approximately 20 to 29 % was achieved, depending on the presence or absence of the bearing tilting effect. In addition, excessive belt tension should be sufficiently considered when designing the shaft, and the FEM analysis results show that the ultra-high-cycle fatigue failure of the fan shaft occurred due to a bending load of approximately 4,000∼5,000 N. The newly replaced shaft was found to operate without problems for more than 1.284×109 cycles by adjusting the belt tension and shaft step radius.