Medium and high voltage direct current (DC) transformer is one of the key equipments for DC collection of new energy generation and voltage conversion in DC transmission and distribution system. Modular multilevel DC transformer (MMDCT) has high modularity and strong voltage expansion ability, which is the mainstream technical solution of high voltage and high power DC energy conversion, showing a rapid development trend. The typical application scenarios of medium and high voltage DC transformers are summarized, and then the typical topology of MMDCT is summarized. On this basis, the key technical problems of MMDCT such as modulation strategy, capacitor voltage balancing control method, circulating current suppression strategy, power control scheme, prototype and application, are analyzed and summarized. Finally, the development trends of MMDCT in the future are prospected, and the multi-functional roles and the multi-direction technical challenges of MMDCT in the future DC power grid are summarized.