Scanning near-field optical microscopy in the infrared and terahertz ranges has attracted much interest in studying objects far below the diffraction limit， particularly in the detection of optical properties of structures on the nanoscale. To further understand the tip-sample interaction， analytical and numerical description of the near fields from the probe is essential. Here， we established and analytically solved a more realistic analytical model based on the real probe shape. Based on the combination of the analytical model and numerical simulation to develop the source dipole model （SDM），a comparison with the new method to full-wave simulation （FWS） results was performed. In simulations combined with the theoretical model， the detection information is obtained directly， and the computational efficiency is improved. Based on simulation results， the antenna effect， tip apex radius influence， and the influence of charge amount are explained. This paper provides a new perspective to further understand the tip-sample junction in optical nanoscopy.