Electrical impedance tomography (EIT) is a technique that calculates the distribution of electrical impedance of a living body by measuring the surface voltage of the living body, which is available for continuous monitoring of the lung function to reduce the risk of intensive lung injury. The global inhomogeneity index (GI) is employed to assess the degree of ventilation homogeneity at different levels of lung injury. The GI values calculated in the current research are all based on two-dimensional EIT reconstruction images; however, this method does not correctly detect low levels of lung injury. In this paper, we establish a true 3D thorax model and propose a method for calculating the GI based on 3D EIT reconstruction images to evaluate lung injury. Simulations reveal that this method can accurately reflect the disease state of lung injury compared with the 2D GI calculation method, and even mild damage of lung injury can be adequately detected.