The transient characteristics of AC/DC hybrid half-wavelength transmission system is quite different from conventional hybrid system. When an asymmetric fault occurs along the half-wavelength line of the hybrid system, not only will the power-frequency overvoltage be formed along the line, but also the dc side will be impacted by over-voltage and over-current. In view of the above phenomenon, this paper analyzes the propagation process of the fault along the half-wavelength line of the hybrid system in detail, and studies the coupling mechanism of fault propagation. In order to suppress the influence of fault effectively, an improved over-voltage limiter is installed on the dc bus and a bi-directional thyristor is connected in parallel at the interface of the converter station to cut off the fault propagation channel. Finally, the simulation model of AC/DC hybrid half-wavelength system was built on the PSCAD simulation platform, and the voltage and current changes before and after the system protection were compared to verify the feasibility of the protection strategy.