The construction of a new power system is an important part of achieving “carbon peak and carbon neutralization target”. The converter transformer plays a key role in the UHVDC (Ultra High Voltage Direct Current) transmission of clean energy, which directly affects the transmission reliability. Core loss is an important parameter for analyzing transformer operation, which is mainly divided into hysteresis loss and eddy current loss. Eddy current loss is generally small. Most of the current studies focus on hysteresis loss, but eddy current loss is an electrical parameter that cannot be ignored for UHV converter transformer. In this paper, a single-phase transformer model is constructed to study the influence of core lamination thickness and frequency on eddy current loss. The fitting equation of core lamination thickness and eddy current loss under normal working conditions is proposed. The combined influence of core lamination thickness and frequency on eddy current loss is analyzed. Eddy current loss decreases with the decrease of sheet thickness at low frequency, but the effect is small, and the main effect is from frequency. Eddy current loss increases with decreasing thickness at high frequency, and the effect becomes more significant with increasing frequency.