The CLLC resonant converter is widely used in the hybrid AC/DC microgrid as a DC transformer to interlink the AC and DC bus, because of its advantages of high power density and the capacity of bidirectional power transfer. In the hybrid AC/DC microgrid applications, as the voltages of AC and DC bus are controlled by the energy management system, the CLLC resonant converter operates under open-loop condition as a DC transformer. This paper mainly focuses on the parameters design of the CLLC resonant converters, with fully consideration of the parameter variations caused by the real working conditions, such as the operation power and ambient temperature. A circuit design method for the CLLC resonant converter to counteract the side effect caused by the working conditions is proposed. With the proposed method, the open-loop CLLC resonant converter can enjoy favourable power transmission and voltage regulation ability although the real values of the resonant inductances and capacitances deviate from the designed values. The proposed method is validated by the experiments.