Fiber differential is the optimal choice of multi-terminal lines, while there’re some problems in application. In this paper, the characteristics of fiber multi-terminal differential protection are introduced, which meet the needs of distribution line transformation. The difficulties and key techniques in actual implementation process of the transformation project are correspondingly analyzed in order to find a reasonable program to solve these problems. Introduction Most transmission lines are two-terminal, which from one end to the other. While for some reasons, some transmission lines are more than two terminal. Especially, three terminal line is called T connection or three-terminal line [1] . And three-terminal lines are very common in distribution network. For multi-terminal lines protection, if there are high standard for speed and selectivity, the fiber differential protection is optimal choice. But in multi-terminal lines even fiber differential protection also exist some problems, the difficulties and key techniques in actual implementation process of the transformation project are correspondingly analyzed in the paper to find a reasonable program to solve these problems. Problems of Fiber Differential Protection in Multi-terminal Lines It is commonly agreed upon that fiber differential protection is optimal choice for multi-terminal line. But in actual implementation process, there are still some factors will decrease correctness of multi-terminal differential protection [2] . (1) Effect of CT saturation in fault condition. The fundamental of differential protection is Kirchhoff’s current law, and when CT is saturation, there will be considerable difference between from measured current and actual current, and big errors may cause mal-operation of the protection. [3] (2) Effect of direct earth capacitance. Earth capacitance will cause capacity current between lines and earth, and unbalance currents will occurs which influence differential protection. (3) Effect of shunt reactor. Just like earth capacitance, shunt reactor also consist current loop which cause unbalance current. (4) Effect of loss of one or more terminal for multi-terminal differential protection. The effect mainly influence criterion, and the influence vary with different criterion. Perhaps quit of some terminal has very little influence to protection which should be considered differently [4] . (5) Effect of synchronous manner of multi-terminal data. Multi-terminal differential protection demand multi-terminal data synchronous transferring which guarantee the correctness of differential operation. Without data synchronization, the multi-terminal differential protection can not operate properly [5] .