Terahertz near-field scanning imaging system breaks through the limitation of the diffraction limit of the traditional far-field optical system and can achieve super-resolution detection and imaging in terahertz band. Based on the working principle of a scattering-type terahertz scanning near-field optical microscope, we established a numerical simulation model of a scattering-type terahertz scanning near-field optical microscope based on COMSOL and verified lightning rod effect and antenna resonance effect generated by the probe excited by incident light, as well as the dipole effect between the probe and the sample. On this basis, we implemented near-field signal calculation and near-field scanning imaging of protein molecules with reference to actual scanning processes. The results show that the model can achieve near-field imaging of three-dimensional materials and has application potential in the study of complex three-dimensional micro nano materials.