For conventional semi DAB DC-DC converters, the turn off current for some switches are pretty high. Besides, ZVS cannot be ensured within the full operating range with constant switching control. EMI is relatively high due to the sharp change of the current in transformers. To deal with the issues brought by conventional semi DAB converters, a resonant semi-DAB-based single-stage high frequency isolated AC-DC converter is proposed. Its multi-mode control strategy with smooth transition is presented. Four decoupled resonant inductor current modulation modes with fixed switching frequency control strategy are given. The resonant tank is designed in an optimized way for better performance and higher conversion efficiency. The converter could get a wider output voltage range with the proposed control method. The primary side switches can achieve ZVS within the full operating range with the proposed fixed switching frequency control. In addition, the RMS value of the resonant inductor current, the maximum current stress of switches and the turn-off current are compared with other modulation strategies under different operating conditions. The power transfer ability, mode transition mechanism and control implementation are also analyzed. A 1000 W experimental prototype was built to verify the effectiveness of the proposed control.