In radiation therapy, the respiration may cause lung deformation. The subsequent deformation and spatial movement of tumors on the lung adversely affect the accuracy and safety of radiation therapy. By combining computed tomography (CT) image 3D modeling technology and respiratory mechanics principle, a new method of lung dynamic deformation modeling and deformation prediction based on a massless spring mechanism is proposed in this article. First, a lung model is formulated, which is based on medical image information before radiotherapy. Then, the deformation of the lung and the movement of the tumor on it are predicted based on respiration. The experimental results show that the lung deformation obtained by this method is consistent with the deformation observed under continuous medical imaging. The error between the predicted value and the observed value is within a reasonable range. This study can accurately and efficiently predict lung deformation and mechanical properties under the influence of respiratory movement under conventional medical conditions. It provides theoretical and method support for subsequent research on tumor displacement compensation.