Abstract: The problem of line protection is the bottleneck of the development of a flexible DC grid. As the theoretical basis of the line protection, the equivalent circuit and transient characteristics after line fault need systematic analysis. This research will provide the cornerstone for the design of new line protection. Using the transient model of the converter, model of transmission line, and the boundary conditions of the fault point, the transient equivalent circuit after decoupling into line-zero mode is established. The transfer functions of the fault traveling wave at the faulty line terminal, the DC bus, and the healthy line terminal are obtained, and the transient characteristics of the initial fault traveling wave are analyzed. Besides, the multiple refractions and reflections of fault traveling waves between inductive line terminal and fault point are analyzed, the change law of the waveform after refraction and reflection is obtained, and the aliasing phenomenon between line-mode and zero-mode at fault point is analyzed. Finally, A four-terminal simulation model built in PSCAD/EMTDC demonstrates the correctness through the comparative analysis of the calculation and the simulation value. The characteristics of the waveform are analyzed when the parameters of the converter and current limiting reactor are changed.