In the multi-terminal DC power transmission system， DC circuit breakers contribute to the fast fault clearing， but the immature manufacturing process leads to the difficulty of its engineering application. This paper uses AC circuit breakers on the DC side of the DC transmission system instead of DC circuit breakers， and a scheme of multi-terminal DC power system control and protections for the DC line fault is proposed. Using PSCAD/EMTDC， a±800 kV bipolar four-terminal DC transmission system module is built and simulated. The simulation result shows that the system under the fault can achieve fast recovery by using the control and protection scheme based on AC breakers. The demand of power transmission is satisfied and the stability of the connected AC system is enhanced effectively.