The equivalent circuit and phasor diagram of inter-phase short-circuit fault of current transformer（CT）secondary circuit are proposed.The effect of short circuit fault via transition resistance in CT secondary circuit on current sampling of protective device and differential protection is analyzed.The results show that the current amplitude of fault phases decrease and the phase angles of fault phases change，which generates differential current in differential protection circuit and may cause differential protection tripping.A case of real differential protection wave is used to validate the correctness of above analysis results.Finally，for such faults，some prevention measures and suggestions are put forward.