This paper investigates the problem of state monitoring of Intelligent Guided Vehicles (IGVs) operating in the complex port environment. Considering the perturbation suffered by the IGV, a two-step interval estimation method by integrating robust observer design with reachability analysis is proposed. First, we physically model the port IGV and its operating states subject to disturbances and noises, respectively. Then, a robust observer for the state monitoring of the considered IGV is designed and the existing conditions of the observer parameters are derived. In addition, based on the conditions for the existence of the proposed observer, the reachable set is obtained by the zonotope technique. Finally, the effectiveness of the proposed two-step interval estimation algorithm for the considered port IGV is verified by numerical simulations.