Abstract: In offshore wind power DC transmission systems, when modular multilevel converters(MMCs) operate with a large number of submodules, conventional finite control-set model predictive control(FCS-MPC) suffers from high computational complexity and degraded transient performance. Moreover, its control structure is not conducive to the implementation of third-harmonic voltage injection, resulting in limited capability in suppressing submodule capacitor voltage fluctuations. To address these issues, an efficient predictive control strategy with embedded third-harmonic voltage injection capability is proposed. The differential-mode voltage and common-mode voltage are introduced as intermediate control variables, whose reference values are derived based on the predictive model of the system, and explicit analytical expressions of the arm voltage references are reconstructed accordingly. Through this formulation, third-harmonic voltage components can be flexibly superimposed onto the arm voltage references, thereby extending the regulation freedom of predictive control at the control-structure level. Simulation and hardware-in-the-loop results demonstrate that the proposed strategy significantly reduces computational complexity while maintaining transient response performance comparable to that of optimal level model predictive control, and effectively suppresses submodule capacitor voltage fluctuations, verifying its feasibility and superiority for offshore wind power MMC systems.