A modular multilevel converter system, and a voltage detection method and an open-circuit fault diagnosis method thereof are disclosed. The modular multilevel converter system includes: N sub-modules sequentially cascaded, where N is an integer greater than or equal to 2, each of the sub-modules comprising at least one bridge arm and a capacitor connected in parallel to the bridge arm, the capacitor having a positive terminal and a negative terminal; and a voltage detection unit for detecting a voltage between at least two adjacent sub-modules in the N sub-modules, the voltage detection unit being connected between the positive terminal of the capacitor of the first sub-module and the negative terminal of the capacitor of the last sub-module in the at least two sub-modules.