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A 4-SPS/U four-level rigid trunk mechanism, with SPS as the driving branch and U as the middle constrained driven branch, is innovatively proposed in this paper. It can improve the flexibility of hexapod mobile robots in posture adjustment in various motion modes such as walking, water propulsion, climbing, and rolling. Firstly, the screw theory is adopted to calculate the degree of freedom of the 4-SPS/U four-level rigid trunk mechanism. Secondly, a combination of closed vector method and decoupling method of eigenstructure assignment has been applied to constructing the inverse kinematics solution, and deriving the velocity and acceleration models as well as analyzing the high posture flexibility in workspace of the trunk mechanism. Thirdly, theoretical simulation data diagram of the movement branch chain displacement, end height, speed and acceleration of the trunk mechanism are calculated through theoretical numerical examples, and the excellent motion characteristics of the 4-SPS/U type four-level rigid trunk mechanism are analyzed. Finally, a quantitative comparison was made between the theoretical simulation data and the experimental data of the experimental prototype. The error rates of the variation curves of the displacement length, height, velocity and acceleration of the driving branch chain were 0.8%, 0.2%, 0.5% and 0.9%, which verified the correctness and reliability of the theoretical derivation.