The integral model of sympathetic inrush is developed firstly. This model indicates that sympathetic inrush is generated through the change of linkage of running transformer by sudden change in system currents passing through the system resistance. The sympathetic inrush value is decided by the value of system inner resistance and the differential area of system current about time axis in a period. This paper systematically expounds the process of sympathetic inrush， and simulates the process of sympathetic inrush based on the model with actual situation. The factors which effect sympathetic inrush value by changing the transformer's neutral point treatment and the power factor of load etc are analyzed. Finally， several advices for avoiding mal-operation of transformer differential protection due to the sympathetic inrush are presented.