When an AC system operates asymmetrically, unreasonable control parameter values at the converter station can affect the stable operation of the flexible DC transmission system. Therefore, a small-signal model of the converter station with differential flatness control under asymmetric conditions is proposed. Simultaneously, combining the internal characteristics of the modular multilevel converter (MMC) with the dynamic models of various subsystems such as the circulating current suppressor and the DC line, a global small-signal model for the two-terminal flexible DC transmission system is established. Based on this, the impact of the control parameters on system stability is analyzed by the changes in the root locus of the inner and outer loops of the differential flatness controller， and determine their reasonable value ranges. Finally, a two-terminal flexible DC transmission test system with differential flatness control in the PSCAD/EMTDC is built to verify the correctness of the proposed small-signal model and conclusions, providing a theoretical basis for controller parameter tuning in practical engineering.