A model of the beam centroid motion in a linear induction accelerator(LIA) was developed. It calculates the track of the beam centroid under the condition of cell magnet misalignment. The 3D magnetic field distribution is calculated with a coordinate transform. The track of the beam centroid is given by a coupled solution of the beam centroid and envelope equations. Quantitative relation between the transverse displacement of beam centroid and the misalignment of magnets is useful in the installation of magnets and the adjustment of steering coils.