Running hybrid HVDC transmission system in the case of communication failure will face a high security risk. An internal station fault at MMC side without communication may cause the capacitance voltage of the submodule to exceed the safety margin. A new solution is proposed in this paper, in which a bypass thyristor is connected in parallel at the sub-module port. The bypass thyristor will be triggered and a designed DC short-circuit fault will be constructed artificially. The DC short circuit fault will be identified by the DC line protection system at the LCC side, and the GS control strategy will be triggered to suppress the over-voltage of the submodule. Finally, a hybrid DC transmission model based on PSCAD is built, and the effectiveness of the scheme is verified by simulation, which ensures that the hybrid HVDC system can be run safely in the case of communication failure.