In this study, an adaptive DC-link voltage control of a two-stage switching power supply with LLC resonant converter is proposed. With this control method, the frequency variation range can be reduced and the conversion efficiency under different load conditions can be improved compared with the conventional variable frequency control technique. By operating the LLC resonant converter near its resonant frequency, circulating loss as well as the conduction loss can be reduced and optimal design of LLC resonant converter can be made possible. Moreover, no additional auxiliary circuits are required for the proposed method. The operational principle of the proposed control scheme is verified with a 350 W prototyping circuit. The switching-frequency-variation range is reduced from 12 to 4 kHz and the efficiency is improved from 89.4% to 90.2% at full load.