High voltage direct current transmission plays a crucial role in the coordinated allocation of power resources in China, and ensuring its stable operation is of great significance. At present, the research hotspot on small disturbance stability of high voltage direct current systems mainly focuses on the modeling of inverters, while the modeling of transmission lines only uses centralized parameters for equivalence. However, centralized parameter models cannot accurately reflect the harmonic transmission characteristics of transmission lines in the high-frequency range. Firstly a distributed parameter model for overhead lines that considers frequency variation characteristics is proposed，which is simplified by eliminating ground wire effects and inter pole coupling effects. Then, the model is applied to the AC side admittance model of high-voltage DC systems. Finally, the correctness and effectiveness of the proposed model is verified on the PSCAD/EMTDC simulation experimental platform.