The earth fault of converter transformer in Nanqiao converter station of Gezhouba-Nanqiao direct current（DC）system is described，so does the protection action.By briefly introducing DC protection operation principle，the fault characteristics of the converter transformer earth fault and the influence on fault current from converter commutation is analyzed.By analyzing the trouble-wave record figure，the fault characteristics and its location were deduced and then the fault was found out.Response of DC system when earth fault of valve occurs on transformer side is summarized to provide theory basis for fast detection.In the meantime，the defect of strategy to switch on adjacent rectifier-inverter stations pair when fault occurs on inversion side has been analyzed.Fault detection on site further confirms above analysis results.The results are instructional and helpful for the design and maintenance of high voltage direct current（HVDC）station.