Converter transformer is one of the most important equipment in the ultra-high voltage direct current (UHVDC) system, whose safe and stable operation is essential. This paper introduces a new phenomenon of inrush current in the converter transformer which is caused by the asymmetric fault on the valve-side winding of the inverter-side converter transformer. After fault occurs, large DC components flowing through the valve-side winding of the converter transformer which may cause the saturation of the core and result in the occurrence of inrush current which we called fault-induced inrush current. For the inverter side, this kind of fault may result in a commutation failure which further increases the DC component in fault current and speeds up the production of fault-induced inrush current. Firstly, considering different failure time and failure locations, the conducting states of thyristors, characteristics of valve-side winding current when single-phase grounding faults occur on the valve side of the inverter-side converter transformer and the probability of commutation failure are discussed. Then, the mechanism of fault-induced inrush current and its impact on the differential protection of converter transformer is discussed, which shows that the fault-induced inrush current may cause the mal-blocking of the protection when the position of single-phase grounding fault transferred from outside of differential protection zone to the inside. Finally, the correctness of the theoretical analysis is verified by PSCAD/EMTDC simulation.