LLC resonant converters usually use frequency modulation or a combination of frequency modulation and phase-shift modulation to adjust the output voltage. However, in these control schemes, a variable switching frequency range is required, which complicates the design of magnetic components. Therefore, the method of variable resonant inductance control is used in this paper to make the converter work at constant switching frequency and constant duty cycle. Generally, variable resonant inductors have a certain dead zone; the existence of dead inductors leads to the adjustment of resonant inductors not being smooth. Therefore, this paper puts forward the magnetic-valve variable resonant inductor, and optimizes the variable inductor with the characteristic of gradual saturation of the magnetic-valve, which reduces the dead zone of the variable resonant inductor and makes the adjustment smoother. And it was verified on a 575 W experimental prototype, whose input is 360 V–380 V and output is 12 V.