The LLC resonant converters commonly adopt frequency modulation (FM) or combination of FM with phase-shift modulation to regulate its output voltage. However, in these control schemes, a variable switching frequency range is required, which makes the magnetic components design complicated. Therefore, in this article, magnetic control (or variable inductor control) is adopted to make the converter operating at constant switching frequency and constant duty cycle. The fundamental harmonic analysis is commonly used because of its characteristic of simplicity. However, the accuracy of this method is reduced and considerable errors occur when the switching frequency or output power changes. Therefore, an optimal design methodology based on time-domain analysis of the LLC resonant converter with magnetic control is proposed in this article. The proposed methodology can assure that the converter will operate in PO or OPO modes within the whole operating range, and zero voltage switching operation for primary switches and zero current switching operation for secondary rectifier will be guaranteed. In addition, by limiting the resonant tank root-mean-square current, the system efficiency is improved. A 200-W experimental prototype is built and the effectiveness of the proposed optimal design methodology is verified.