The modular multilevel converter(MMC) can be applied to the situation of high voltage and large capacity. With the increase of sub-modules(SMs), substantial isolated voltage sensors are required in the control of circulating current suppressing and capacitor voltage balancing. Consequently, the system complexity is increased, and the system reliability is reduced. Therefore, an estimation method of capacitor voltage is put forward based on sliding mode observer(SMO) to simplify the hardware structure of SMs and to reduce the utilization of voltage sensors. Among which, the voltage sensorless control is carried out with an estimated capacitor voltage, and the balance of capacitor voltage is maintained by fuzzy control. The effectiveness of the proposed method in MMC sensorless control is verified by experimental results, which is robust to capacitor parameters and load changes.