<p>The fundamental harmonic analysis (FHA) method ignores the high harmonics over a wide gain range, which leads to large errors and low precision in the voltage gain, particularly at the point where deviation from resonance occurs. A multi-objective parameter optimization method of LLC based on the time-domain analysis (TDA) is proposed. Through analyzing the optimal operating mode (the PO mode) of half-bridge LLC resonant converter in the time domain, the state-time domain equations are established in the PO mode, the correlation coefficients <i>k</i>₁, <i>k</i>₂, <i>k</i>₃ and <i>k</i>₄ of the state equations are obtained. Meanwhile, according to the load constraints, the coefficient constraints and the angle constraints, through deriving the resonant capacitance <i>C</i>_r, the inductance coefficient ratio <i>K</i>, and the conduction angle θ in the sub-interval of P mode, the complete time-domain analysis in PO mode is realized. Based on the multi-objective constraints of mode boundary, the resonant capacitor maximum and ZVS, the parameter design area is greatly optimized. A set of <i>C</i>_r and<i> K</i> optimal solutions are determined in this area to realize overall high efficiency of the converter over a wide input voltage range. Besides, to verify the advantages of the proposed method, it is compared with FHA and simplified time-domain analysis (STDA) in terms of switching frequency range, ZVS operating range, and resonant current. Finally, a 500W experimental prototype of the LLC resonant converter is built to verify the correctness of the proposed theoretical analysis.</p>