The flexible DC transmission system based on modular multi-level converter is a low-inertia and weak-damping system. Once DC short-circuit fault occurs, the rapidly rising fault current will damage power electronic equipment and destroy the safe operation of power grid. In order to study the fault current limiting method, the modular multi-level converter (MMC) is equivalent to a controlled current source according to the power balance principle firstly, and the DC impedance models in frequency domain under different control modes are established considering the ratio of inserted submodules. Then the influence law of converter circuit parameters and control parameters on fault current is analyzed. It is concluded that the key influencing factors are arm inductance and the ratio of inserted submodules. A fault current limiting control strategy based on DC impedance remodeling is proposed to suppress the rise of fault current by adaptively reducing the ratio of inserted submodules. Finally, the accuracy of the DC impedance and the effectiveness of fault current limiting control are verified by simulation.