The invention provides a sub-module topology structure and method for a flexible DC power transmission converter valve. The sub-module topology structure comprises a full-bridge structure sub-module (1) and a half-bridge structure sub-module (2) which are mutually connected, wherein an AC end A2 of the full-bridge structure sub-module (1) is connected with a DC end positive electrode D21 of the half-bridge structure sub-module (2), and a DC end negative electrode D12 of the full-bridge structure sub-module (1) is connected with a DC end negative electrode D22 of the half-bridge structure sub-module (2) through a diode D7. Through an additional diode, the full-bridge and half-bridge hybrid topology is reconstructed into a new topology capable of inhibiting a fault current, in the fault current inhibition process, voltage balance can be automatically kept in two capacitors of the sub-module, and problems of voltage imbalance and the overhigh withstanding voltage can be avoided.