Abstract: Flexible DC transmission technology has developed into a main method of ultra-high voltage (HVDC) transmission. However, the low-frequency oscillation faults occuring in several industrial sites at the same time will affect the reliability of power supply. The existing theories explain the causes of low frequency oscillations under abnormal operating conditions such as transient short circuits from the perspectives of induction generator effects and subsynchronous control interactions, but they cannot analyze the mechanism of low frequency oscillations during normal power transfer. Based on this, this paper systematically studies the low-frequency oscillation problem in the flexible DC transmission system from multiple angles: (1) Quantitative analysis of a mathematical model is given to illustrate its mechanism of action: It is the MMC impedance characteristics, the switching frequency, the low-frequency operating characteristics, and the low-pass characteristics of the circulating current that cause the low-frequency oscillation; (2) Various control methods, such as circulating current suppression, switching frequency optimization and active damping, are used to improve the existing low-frequency oscillation suppression algorithms; (3) The RTDS hardware-in-the-loop simulation of a double- ended ±420kV flexible DC transmission system is built. By using the parameters of the typical Flexible HVDC Transmission Project and comparing the implementation results of the proposed algorithm and the traditional algorithms, the applicability of the industrial site is verified. The low-frequency oscillation mechanism mentioned in this paper can explain various causes of low-frequency oscillations. The proposed control strategy can suppress low-frequency oscillations and improve the reliability of flexible DC transmission systems.