During the back-pulling process of horizontal directional drilling (HDD) pipes, the coating inevitably experiences some degree of damage, compromising the pipe’s overall safety. To address this, cathodic protection is typically employed to provide additional protection to the damaged areas of the coating. However, assessing the effectiveness of cathodic protection in HDD pipes presents significant technical challenges, particularly in accurately determining the polarization characteristics at varying burial depths. This study developed a deep-well polarization testing device and conducted field tests on an actual oil pipe. The results indicated that at a burial depth of 1 m, the pipe showed an oxygen diffusion limitation zone. In contrast, at depths greater than 5 m, the soil’s oxygen content was extremely low, and no oxygen diffusion limitation zone appeared in the polarization curves. As the burial depth increased, the pipe’s polarization characteristics underwent substantial changes. This device offers a reliable method for measuring cathodic protection potentials at different depths in HDD pipes, providing a scientific foundation for establishing cathodic protection evaluation standards and optimizing design parameters.