This study aims to explore the current field distribution characteristics and detection influencing factors of the focusing DC induced polarization (IP) method for tunnels. The study processes are as follows: the normal and anomalous potentials of the spatial electric field were calculated using the finite element method; a homogeneous 3D geoelectric detection model was constructed using the Comsol software, the model was divided into grid cells using the adaptive algorithm for tetrahedral mesh generation, and the numerical calculation precision was compared and analyzed; the distribution and change patterns of the focusing current field were investigated, and the range of the current ratio of the focusing effect was determined, and a 3D geoelectric model was constructed for anomalous geological structures in front of the tunneling section, and the influences of factors such as the area of the tunneling section, interference bodies, and current ratio coefficient of focusing effect on the IP effect parameters were simulated through the forward modeling using the equivalent resistivity method. The results are as follows: the focusing DC IP method was highly sensitive to the detection of the unfavorable geological bodies in front of the tunneling section; the impact of the interference bodies in the tunnel cavity on the detection of the anomalous target bodies can be ignored; the farther the lateral anomalous interference bodies from the tunnel floor, the less the impact on the detection of the anomalous target bodies, and the exploration distance can be effectively increased by increasing the area of the tunneling section and the current ratio coefficient of the focusing effect. This study can be utilized as a basis and reference for the inversion using the focusing DC IP method and is greatly significant for promoting the development of the focusing electrical exploration theory.