Sympathetic inrush may cause mal-operation of transformer or generator protection. Effective measures to avoid the mal-operation of differential protection caused by sympathetic inrush are still not found. The physical mechanism of emerging sympathetic inrush is analyzed. According to the characteristic of sympathetic inrush， mal-operation of differential protection should not be directly caused by sympathetic inrush. Due to 2nd harmonic restrain theory widely used in practice， its adaptability under different transferring characteristics of current transformer while sympathetic inrush happens is studied. It is concluded that current transformer partial transient saturation is one of the most important factors that results in mal-operation of differential protection. At last， one modified ratio restrain scheme for differential protection is proposed so that the mal-operation caused by current transformer partial transient saturation can be avoided.