The UHF method is an effective method for detecting partial discharges (PD) in transformer bushings, which enables the identification of discharge types by extracting characteristic information from their waveforms, however, the waveform distortion occurs when the UHF signal passes through the capacitor core, which has a serious impact on the identification of discharge defects. Based on this, this paper proposes a study of the distortion characteristics of PD electromagnetic signals in capacitive bushings. A simulation model is established in accordance with the capacitive core of a real transformer bushing, and the finite-difference time-domain (FDTD) method is used to study the signal distortion generated by the PD electromagnetic wave signal passing through the capacitive core, as well as the distortion law of excitation source signals with different pulse widths and pulse amplitudes when passing through the capacitive core. The simulation results show that the UHF signal passing through the capacitor core causes its pulse width to become larger and pulse amplitude to become smaller; the excitation current source pulse amplitude remains the same, the larger the pulse width, the smaller the signal distortion; the excitation current source pulse width remains the same, the pulse amplitude increases, the pulse amplitude of the signal at the observation point increases in equal proportion, while the pulse width does not change significantly. The simulation results in this paper provide a reference for identifying the type of discharge when there is distortion in the UHF waveform in the capacitive transformer bushing.