One of the promising applications of Electrical Impedance Tomography (EIT) is to monitor lung ventilation. However, during respiratory activity, chest moves as much as 10% in the anterior-posterior direction. No EIT reconstruction algorithm has considered this change. For EIT system, which is sensitive to electrode positions, whether chest expansion needs to be incorporated becomes an important problem to study. The results using a 2D thorax model showed that chest expansion accounted for up to 20% of the reconstructed image amplitude and introduced an artifact in the center of the image. To better characterize this issue, a 3D model with detailed anatomical structures, which was developed based on a series of MR thorax images covering from neck to abdomen, was used to simulate the influence of chest expansion. At one selected layer of the model, sixteen electrodes were placed evenly to simulate real measurement setup. Chest expansions were simulated by changing the size of 3D human thorax model. Then EIT dynamic images were reconstructed based on stimulated measurements before and after chest expansion. By analyzing these images, the relationship between chest expansion and the amplitude in the lung regions was established. The results show that chest expansion accounts for up to 31% of the changes in the reconstructed resistivity images. And they further confirm that chest expansion introduces an artifact in the center of the images. Based on this study, chest expansion should be considered in EIT image reconstruction.