The present invention discloses a modular multilevel dynamic switching DC-DC transformer, which consists of N DC-DC sub-modules connected in series. Each of the DC-DC sub-modules is of a symmetrical structure capable of realizing bidirectional power flow, which consists of two half-bridge sub-modules and isolating switches S1, S2 between capacitors C1, C2 of the two half-bridge sub-modules. The capacitors C1, C2 are disposed in parallel between the half-bridge sub-modules and the isolating switches. Assuming that the power flows in the primary side of the DC-DC transformer and flows out from the secondary side; the half-bridge sub-module located on the primary side of the DC-DC transformer is referred to as a primary-side sub-module; and, the half-bridge sub-module located on the secondary side of the DC-DC transformer is referred to as a secondary-side sub-modules. By turning on or off the switching devices, the switch-on or switch-off of the primary-side sub-module or the secondary-side sub-module can be realized, leading to the dynamic switching in four different operating states of the DC-DC transformer and realizing the voltage conversion and energy exchange. Compared with the prior art, the present invention is able to change the transformer ratio dynamically, realizing the control of the DC power flow, and, greatly decreases the number of sub-modules, has remarkable economic efficiency, can effectively reduce the size of DC-DC transformers, and it is advantageous for popularization.