This paper introduces a new MMC topology, in which each phase leg consists of four arms. Two of the arms consists of cascaded connected submodules and are just like single phase leg in traditional MMC topology. Another two arms are formed by series switches such as high voltage IGBT, which are only activated when the ac voltage changes polarity. And then three phase legs are series connected in vertical direction to support the DC bus voltage. The major advantage of proposed topology are as follows: (1) The total number of required submodules are reduced to 1/3 compared with traditional MMC topology. (2) The energy ripple in each arm is calculated to be smaller than the traditional 3-phase MMC. Therefore, the total capacitor size can be reduced. A detailed 20 MW simulation has been conducted and the related results of proposed topology have been presented. What's more, The SM prototype and preliminary experiments are also demonstrated and analyzed.