After a single-phase ground fault of the inverter side converter side of a UHV DC transmission system, the voltage drop on the grid side may cause commutation failure of the DC system, such that the DC component flows into the converter transformer winding, and causes the transformer core flux to shift, causing the converter transformer to become saturated, causing a faulty inrush current, which in turn affects its safe and stable operation. In this paper, the problem of fault inrush current is generated for the single-phase ground fault of the inverter side commutation network. The characteristics of the grid side fault current under the two conditions of normal commutation and commutation failure of the converter valve are discussed using the theory of switching function. The variation of commutation flux after a single-phase ground fault on the side of the side network is studied. On this basis, the influence of current transformer saturation on commutation differential protection is studied. The results show that the grid-side current transformer is saturated under the fault condition in the side of the converter network, so that the second harmonic content of the commutating differential current exceeds the threshold value, resulting in differential protection mis-locking. Finally, the correctness of the analysis is verified by PSCAD/EMTDC simulation. This work is supported by Key Project of Smart Grid Technologies and Equipment of National Key Research and Development Program of China (No. 2016YFB0900600) and Science and Technology Project of State Grid Corporation of China (No. SGNX0000DKJS1900460).