When the conventional half-bridge modular multilevel converter based HVDC (MMC-HVDC) transmission technology is applied to overhead UHVDC transmission lines, it confronts challenges such as difficult to isolate DC faults, unable to operate under reduced DC voltage, and unable to realize the on-line bypassing/insertion of valve group. To overcome the above challenges, this paper presents a non-blocking overhead MMC-HVDC transmission technology. The proportion design theory of full-bridge power sub-module in the overhead MMC-HVDC transmission system is described in detail, and the control architecture of the MMC-HVDC transmission system is redesigned. On the basis of the traditional dq axis inner loop current control, the DC current control inner loop is added. The requirements of the new control architecture and the inadaptability of the conventional half-bridge control for the overhead MMC-HVDC transmission system are discussed. The new control architecture is described, and the validity of the no-locking control is verified by experiments.