The modular multilevel converters (MMCs) are popularly used in high-voltage direct current (HVDC) transmission systems. However, for the direct modulation based MMC, its complex internal dynamics and the interaction with the grid impedance would induce the frequency coupling effect, which may lead to instability issues， especially in the case of weak grid. To effectively suppress the sub- and super-synchronous oscillations, this paper proposes a linear active disturbance rejection control (LADRC) based MMC control strategy. The LADRC mainly consists of the linear extended state observer (LESO) and the linear state error feedback (LSEF). And it is a potential method to enhance the system stability margin, attributing to its high anti-interference capability and good tracking performance. Thereupon, the system small-signal impedance model considering frequency coupling is established. And the effect of the introduction of the LADRC on the system stability is further investigated using the Nyquist criterion. Particularly, the influences of key control parameters on the stability are discussed in detail. Meanwhile, the impact of LADRC on the transient performance is explored through closed-loop zero poles. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified via electromagnetic simulations.