With the rapid development of UHVDC transmission engineering, the grounding current of converter transformer cores and clamps has attracted more and more attention from scholars. When there is an additional fault grounding point in the transformer core, the grounding current will increase rapidly, and the clamp grounding current contains a large number of harmonics. In this paper, a high-precision ground current monitoring system is designed for time and frequency domain detection. Then, a fault diagnosis method based on PNN was proposed. Finally, the comparative experiment of grounding current frequency domain analysis is carried out, and the experimental results show that the measurement error of the designed system is less than 5%, which meets the actual operation and maintenance requirements. The grounding current monitoring and diagnosis device of converter transformer core and clamp can accurately monitor the grounding current and provide reliable diagnostic results, provide on-site guidance for the operation and maintenance personnel of the converter station, and have significant engineering application value.