Abstract: The photovoltaic（PV） DC boosting integration system should have the ability of fault-ride-through（FRT） when the AC grid fails. However, the control strategy of DC transformer（DCT） is difficult to automatically switch back to its original maximum power point tracking（MPPT） control when the fault by large transition resistance is cleared. A fault clearing coordination control strategy based on modular multilevel converter（MMC） second harmonic injection was proposed. By correcting the negative sequence current reference of converter, the second harmonic is actively injected to the system when the fault is cleared, and DCT can switch back to the MPPT control in time by monitoring the amplitude of the DC second harmonic voltage. The PSCAD simulation results show that the control strategy of DCT can reliably and quickly switch by using the proposed method under various fault types of the host grid, which effectively improves the utilization rate of the photovoltaic.