Abstract: The article presents an improved three-level LLC resonant soft-switching DC-DC converter. By introducing a bus-symmetric flying resonant capacitor and a clamp diode, the system stability is enhanced, and the system setup was validated. A significant drawback of conventional converters is the heat generation and voltage stress issues caused by the current passing through the resonant capacitors, which poses a risk of system instability.I proposed the method of using a bus-symmetric flying resonant capacitor along with an additional clamp diode to achieve soft-switching control under a wide range of output voltage conditions. Experimental results indicate that the proposed method effectively suppresses voltage overshoot during the startup phase, reduces capacitor heating,and allows the low-voltage resonant capacitor to lower system costs while enabling dynamic adjustment of the output voltage with good dynamic response.