In the future, the capacity expansion of the transmission lines is an important topic in the power system research. In this paper, a new conception of the capacity expansion for the LCC-HVDC transmission lines is proposed. The original FACTS devices of the LCC-HVDC transmission can be dismantled and the modular multilevel converters (MMC) of the MMC-HVDC transmission will be built on the empty land. The original HVDC transmission towers can be dismantled and the “double circuit DC transmission towers” will be built. The conception is equivalent to building a MMC-HVDC transmission line in parallel to the LCC-HVDC line. This topology can not only expand the capacity, but also solve the problems of the reactive power and the harmonic current on the AC side. This paper also proposes a control strategy for the MMC in the MMC-HVDC. The simulation results show that the proposed control strategy can perfectly meet the requirements of the capacity expansion and the compensation. Furthermore, for the problem of the zero sequence circulating current in the parallel operation, a simple suppression strategy is proposed. The zero sequence circulating current is well suppressed.