The forces in Double Ring-and-Trunnion Universal Joint arc complicated, especially the middle shaft having varied loads. The dynamic performance of shaft directly affects the performance and time being used under high speed rotation. Based on the multi-flexible dynamics, the shaft in the joint system was flexi-fied. The flexible-rigid coupling model was created, dynamic analysis and simulation were conducted to make it closer to actual working condition. Calculation results will provide necessary basis for design and application of the shaft in ring-and-trunnion joint.