DC transformer with CLLC resonant network is the core component of mobile power supply, and its performance is affected by external environment and component parameter errors, leading to the voltage ratio fluctuation. The characteristics of the relationship between the normalized voltage gain and the normalized switching frequency of CLLC resonant DC transformer (shorted in DCT) is analyzed in this paper. A resonance frequency automatic tracking method based on maximum gain point search is proposed, and a frequency tracking control algorithm and its control system are developed. A simulation model and also an experimental system was used to carry out automatic tracking control experiments of resonant frequency under two conditions of initial under resonance and over resonance. The results show that the frequency tracking control based on maximum gain point search effectively achieved automatic tracking of switching frequency on resonant frequency. The DCT operated in a quasi-resonant state, and the voltage ratio remained constant.