The rotating body of shield tunnel segment assembly machine cracks or even malfunctions due to shear force and impact when applying in shield tunnel segment assembly. To address this, the causes for failure of the rotating body are analyzed, focusing on the structure and working mechanism of the segment assembly machine. The analytical results indicate that the segment assembly machine is subjected to the impact load of segment grabbing and rotating and the static load of segment pretightening stress. A simplified mechanical model is employed to examine the stress state of the rotating body under impact and static loads, and statics and dynamics simulation validations are conducted using finite element method. The research results demonstrate the following: (1) The rotating body is subjected to bending moment and shear force generated by the selfweight of the segment during the working process of the segment assembly machine. The causes for cracks or malfunctions of the rotating body are uneven force, large impact load, random impact frequency, unreasonable structure design, and nonstandard operation. (2) Under working load condition, the stress and torque of the rotating body peak when rotates 90° from the initial position. When applying an excessive rotating speed, the rotating body is prone to damage and deformation due to large impact force, which is consistent with the stress analysis results. Based on the analysis and simulation results, a diaphragm is welded to strengthen the stress concentration position of the rotating body, and the effect is numerically simulated. This countermeasure is proved to be effective in preventing the rotating body from cracking or malfunctioning.