Abstract: Current source converter (CSC) is a new type of current source converter, which has the advantages of high power density, flexible control, and no commutation failure risk, and it also has broad application prospects in the field of DC transmission. CSCs using pulse width modulation have problems such as large DC voltage fluctuations and high switching losses, and are not suitable for DC transmission scenarios. Although the CSC of fundamental frequency modulation alleviates DC voltage fluctuation, it only has one degree of control freedom, and cannot realize active and reactive independent control. In view of the above problems, this paper proposes an improved fundamental frequency modulation current source converter suitable for high-voltage DC transmission on the basis of fundamental frequency modulation. First, the principle of improved fundamental frequency modulation and the harmonic characteristics of the AC and DC side of the converter are analyzed, and the DC filter is designed. Then, the mathematical model of the system is established, and the corresponding system power control strategy is proposed. Finally, a simulation model is built in PSCAD/EMTDC to verify the correctness and effectiveness of the proposed modulation method and control strategy.