The capacitor-inductor-inductor-capacitor (CLLC) resonant converters are drawing more and more attention for their superiority in soft switching, wide output range, and symmetrically bidirectional operation. However, CLLC converter still suffers the problems of unsatisfactory voltage regulation and low efficiency under light-load conditions. Therefore, this article proposes an optimization for extended phase-shift (EPS) control in full-bridge CLLC resonant converter to improve the light-load efficiency. In order to study the relations between phase-shifts of EPS control and converter efficiency, a detailed circuit model is first established to solve the voltage gain, time-domain expressions of main circuit variables, and root mean square (rms) values of resonant currents. Then, a comprehensive loss evaluation is conducted by calculating and analyzing the main power losses, including conduction loss, switching loss, and core losses of magnetic components. Besides, zero voltage switching conditions of primary and rectifier switches are derived to define the range of soft switching. Finally, based on these analyses, the optimal combination of phase-shifts for EPS control is determined to achieve the maximum conversion efficiency of full-bridge CLLC converter at light-load conditions. The validity of the proposed optimized EPS control is verified on a 18~25 V/400 V, 200 W GaN-based CLLC resonant converter prototype.