Abstract: Non-common-ground two-phase interleaved Boost DC-DC converters (NCG-TIBDCs) have been widely applied into the power converters with the requirements of low current ripple and high voltage gain. An active flexible start (AFS) method was proposed to eliminate the start-up surge current of the NCG-TIBDCs. To implement the proposed AFS, an active branch was added and paralleled with the input inductor of NCG-TIBDCs. Furthermore, a coordinated modulation strategy for the switch in active branch and the main switches in NCG-TIBDCs was also proposed. With the proposed AFS method, the overshoot of the start-up surge current can be completely eliminated. In detail, the mechanism of the start-up surge current in NCG-TIBDCs was analyzed firstly, then the idea and method for realizing the AFS was proposed, and the capacitor clamped NCG-TIBDC was taken as the example for the theoretical, simulation and experimental verification. By comparing the direct start, the programmed soft start and the proposed AFS, the effectiveness and advantage of the proposed AFS on suppressing the start-up surge current was proved. Compared with the programmed soft start, the AFS method can completely eliminate the start-up surge current spike and overshoot. The proposed AFS method can be employed into NCG-TIBDCs with different types of Boost structures, which has good application prospect in NCG-TIBDCs.