The invention discloses an optima design method of an LLC resonant converter. The method comprises the following steps: determining transformer ratio of transformation according to a design index of an LLC resonant converter; determining the primary side turns, the secondary side turns, a winding structure and a magnetic core shape of the transformer according to the ratio of transformation of thetransformer, and initially designing a transformer; measuring a parasitic capacitance of the transformer; deducing the relation formula of switch-on loss and dead time to obtain the optimal dead timewhen the switch-on loss is minimum; determining the optimal magnetic inductance according to the optimal dead time; selecting the inductance ratio and quality factor capable of satisfying a gain condition; and obtaining a resonance inductance value and a resonance capacitance value according to the optimal magnetic inductance, the inductance ratio and the resonance frequency; and changing the airgap of the transformer to enable the magnetic inductance to be equal to the optimal magnetic inductance. Through the method disclosed by the invention, the problem that the traditional LLC resonant converter design method depends too much on the experience and is inaccurate is solved, the design result is more accurate, and can be popularized to various application occasions.