Converter transformer is the most important equipment in HVDC system, and its differential protection plays a significant role to guarantee the safety operation of HVDC system. At present, the second harmonic restraint method is usually adopted in converter transformer differential protection for blocking inrush currents. The converter adopted in ultra HVDC system is non-linear component, which results in differences between protective performance of the converter transformer and the common power transformer. This paper makes detailed analysis on the different fault characteristic of the converter transformer by constructing relevant short circuit current flow path, which includes the internal single phase to ground fault, internal phase to phase fault, and the valve fault in rectifier. The research results show that the differential protection of converter transformer will not mal-operate during an external fault, but it may refuse to operate when internal faults. This work is supported by Key Project of Smart Grid Technology and Equipment of National Key Research and Development Program of China (No. 2016YFB0900600).