It is increasingly known that the sympathetic inrush may cause the mal-operation of differential protection for transformer or generator， and affects their normal running. Up to now， because the effective measure is no available to identify the sympathetic inrush， the second harmonic restraint scheme against initial inrush and third harmonic based current transformer（TA） saturation criterion are used to prevent differential protection to mal-operation caused by the sympathetic inrush. Considering the characteristics of sympathetic inrush， it is analyzed that mal-operation of differential protection related to sympathetic inrush is mainly due to the TA saturation. Further， through the mal-operation cases in practice and simulation results， the effectiveness of TA saturation criterion is analyzed under different connections between transformers， such as in parallel， in series and their coexisting. The analysis results verify that the TA saturation criterion can play a certain role in blocking the differential protection for transformer connection in series， but it may be invalid under transformer connection in parallel or parallel and series coexisting.