Abstract: At present, the fault location method of modular multilevel converter (MMC) has many problems, such as difficult fault threshold selection, complicated calculation process and extra detection link. In order to solve the above problems, this paper proposes the error between the theoretical and actual values of submodule capacitance current as a new fault diagnosis feature based on the comparison analysis between the actual and theoretical paths of sub-module capacitance current under the fault state. A model-free and robust least square support vector machine (LSSVM) network is used as a new fault location scheme to realize the fast location of submodule multi-tube open-circuit faults. The proposed method can estimate the current value of a submodule from its capacitance voltage without adding additional current sensors by establishing the submodule capacitance current estimation equation based on the first-order Euler approximation principle. Compared with existing capacitor voltage-based fault signatures, this feature can quickly reflect submodule fault conditions when the capacitor voltage has not deviated significantly from normal values. Finally, the correctness and effectiveness of the proposed fault location strategy are verified by using Simulink simulation and StarSim hardware-in-the-loop experimental platform.