Aimed at the problem that DC-DC converters with coreless transformers must operate at high frequencies, a DC-DC converter with a coreless transformer based on free decay oscillation is proposed. First, a circuit model in series-series topology with no excitation sources is established, the evolution of the eigenvalue is discussed, and the free decay oscillation behavior of the system is analyzed. Second, the DC-DC converter with a coreless transformer based on free decay oscillation is designed. The switching frequency of this converter can be significantly lower than the operating frequency of the coreless transformer, and the output power from the converter and the operating frequency of the coreless transformer can be described by the eigenvalue of the free decay oscillation system. Finally, an experimental prototype was constructed, which was under constant duty control. The switching frequency of the converter was reduced to one half(103 kHz) and one third (69 kHz) of the operating frequency of the coreless transformer (206 kHz), respectively. The converter efficiency achieved 91.2%, and the transformer efficiency was always higher than 96%.