Abstract: Impedance analysis method is widely used to study the stability of modular multilevel converter based high voltage direct current transmission（MMC-HVDC） system. The impedance model of modular multilevel converter（MMC） is the basis of the application of this analysis method. In the practical engineering, the positive-negative sequence current control loops and the sequence separation algorithm are the important components of the MMC control system. The research shows that when the delaycancellation based positive-negative sequence separation algorithm is adopted, the AC impedance of the MMC-HVDC system will fluctuate regularly. However, the mechanism of this fluctuation phenomenon is not fully studied at present. Firstly, considering the positive-negative sequence current control loops and the sequence separation algorithm, this paper deduces the detailed impedance model of MMC, reproduces the impedance fluctuation phenomenon, and then studies the influence of the inconsistency of controller parameters of positive-negative sequence current loop on the fluctuation characteristics. Secondly, the simplified impedance model of MMC is deduced and the rationality of using this model to analyze the impedance fluctuation phenomenon is verified. Based on the simplified model, the generation mechanism of impedance fluctuation phenomenon is analyzed theoretically,and the influence mechanism on the aggravation of AC impedance fluctuation with the increase of the inconsistency of controller parameters of positive-negative sequence current loop is studied theoretically. Finally, experiments verify the correctness of the above theoretical analysis.