The modular multilevel converter (MMC) is widely used in the field of high-voltage and high-power transmission due to its advantage of modular structure. As a large number of submodules (SM) are contained in the MMC,the switch failure of SM is one of the key issues affecting the reliability of MMC. The switch open-circuit fault is usually hard to be detected immediately,which threatens the normal operation of the system. A SM open-circuit fault localization method based on Pauta criterion is proposed in this paper. Firstly,the open-circuit fault characteristics of SM are analyzed in this paper. The SM open-circuit fault causes the abnormal changes of the SM capacitor voltage,which can be judged by the Pauta criterion. Then,a confidence interval is constructed by the average SM capacitor voltage and three times of the standard deviation of SM capacitor voltages in an arm. The SM open-circuit fault can be located through judging whether the SM capacitor voltage exceeds the confidence interval or not for a certain time. The proposed method does not require additional sensors,creation of accurate mathematical models and manual setting of empirical thresholds,and its algorithm is quite simple. Finally,a three-phase MMC is simulated with PSCAD/EMTDC and a single-phase MMC prototype is built in the laboratory to verify the proposed method. The simulation and experimental results both confirm that the proposed Pauta criterion based SM open-circuit fault localization method can locate the SM open-circuit faults effectively and quickly.