Ferrite antenna is modeled by 3D finite element electromagnetic simulation software to achieve near-field communication in complex downhole environments. The results show that the coil radius, wire radius, and antenna spacing have a significant impact on the forward transmission gain (S21) of the antenna, while the influence of the relative permeability of the ferrite loop is relatively small. To investigate the impact of the underground environment on antenna transmission performance, antenna impedance matching is performed, and a drilling tool model with drilling fluid medium is established. The results show that metals cause 3.3 dB attenuation of S21; increasing conductivity of drilling fluid from 0 S/m to 2 S/m causes 8.2 dB attenuation of S21. Simulation results and testing results show great consistency, the deviation of S21 curve is less than 0.3 dB for the basic model and less than 5.5 dB for the extended model, guiding downhole NFC antenna design.