This paper analyzes the primary causes and its possibility of the reversed current in serial compensation lines, according to the model of Jibei Wanshun 500 kV ultra-high voltage transmission line. Besides, the variation tendencies of differential protection ratio coefficient K of serial compensation lines on the condition of at different failure points, degrees of series compensation, grounding circumstances via high resistance and MOV breakdown are discussed by specific cases in this study. Practical variables under current operation mode are taken into simulation by RTDS. The performance characteristics of zero-sequence differential and phase difference protection affected by serial compensation are presented under single/double line operation modes. Since then, the performances of differential protection with variant transient resistance, failure points, systematic variables and load are obtained, furthermore, the improvements of Wanshun serial compensation protection devices are recommended at the end.