The large-scale transfer of DC power flow may cause a converter station to operate outside safety limits. This may cause cascading failures in the DC system. In order to clarify the power flow distribution and transfer characteristics of a DC system under different conditions and its impact on safe operation, first, this paper analyzes the characteristics of power flow transfer after removing the faulty line or changing the injected power of a converter station under master-slave control and droop control. It obtains a quantitative calculation method for power flow transfer in different operational modes. Then, based on PQ power domain, the safe operation boundary of each station in a complex DC grid is analyzed. The rules of different operational and control modes affected by power flow transfer and safety boundary are clarified. Finally, this paper simulates the proposed calculation method and the conclusion of security boundary analysis through the CIGRE DC grid test model. The simulation results based on PSCAD software confirm the correctness of the theoretical analysis. This work is supported by the National Key Research and Development Program of China (No. 2018YFB0904600).