A transmission shaft assembly for use in a sequential test of flexible thin-walled bearings includes a power connecting member, a clamping device and a plurality of bearing mounting kits. The bearing mounting kits are located between the power connecting member and the clamping device. The clamping device penetrates a central bore of each of the bearing mounting kits and is connected to the power connecting member. The power connecting member, the clamping device and the bearing mounting kits are connected into one piece. The power connecting member includes a spindle shank and a blind rivet. The bearing mounting kit includes a bearing mounting sleeve seat, a rear retaining ring, a front retaining ring, a bearing locking bolt and a tightening nut. The clamping device includes a clamping bolt, a washer and a spring washer.