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.