This paper is concerned with the state estimation problem for an automated straddle carrier. An improved set-membership filtering scheme is proposed to solve the state estimation problem of the automated straddle carrier under unknown-but-bounded(UBB) process and measurement noises. First, the kinematic model of the automated straddle carrier is linearized to obtain the vehicle linear dynamic model. Then, an improved filtering method is designed to realize the state estimation of vehicle motion parameters by obtaining the state estimation ellipsoids. Finally, the simulations verify the feasibility and effectiveness of the proposed method. The experimental results show that the proposed algorithm has good state estimation performance.