Abstract: Floating discharge is a common form of the oil-paper insulation defects in the converter transformer. However, in addition to AC or DC electric field in converter transformer, there also exists special AC-DC combined electric field. Therefore, in this paper, by combining with actual operation environment of converter transformer, a set of AC-DC combined partial discharge detection platform was built. The pulse waveform of oil-paper insulation floating discharge under AC-DC combined electric field is obtained by using of series compression method, and the union features of waveform in time domain, frequency domain and time-frequency domain are extracted. The experimental results show that the floating discharge of the converter transformer generates first primary pulse discharge waveform with two different forms, and each kind of waveform has obvious time-frequency characteristics, which can be used to diagnose floating discharge of the converter transformer.