The DC distribution network has great advantages on improving the reliability, power quality, and accessing the distributed power and DC load. However, when a fault occurs, the DC current will rise fast and have great peak value, and it may damage the converter and insulation equipment. So, the DC line fault handing strategy is strictly required. MMC based DC distribution network can achieve line-ground fault crossing via designing a proper grounding method, but there is still no method to handle the line-line fault. In this paper, the transient characteristics of line-line fault and its impact on AC and DC system is studied in a back to back DC distribution network. A line-line fault control and protection strategy is proposed and its cooperation with main equipment parameters is researched. Simulation is conducted on PSCAD/EMTDC, and the results verify the correctness of control, protection and cooperation method. This work is supported by National High-tech R & D Program of China (No. 2015AA050103).