The invention discloses a direct-current monopolar grounding fault detection method for a flexible direct-current transmission system. Turn-ground voltages of direct-current buses and current change rates of the positive and negative direct-current buses are obtained as direct-current monopolar grounding fault judgment parameters; when duration that an absolute value of the turn-ground voltage of one pole is smaller than a voltage comparison limit value Udcset exceeds a time limit value Tset, the duration that the absolute value of the direct-current change rate of the pole is greater than a current change rate comparison limit value dIset exceeds the time limit value Tset, and meanwhile, the duration of the absolute value of the direct-current change rate of another pole is smaller than a second current change rate comparison limit value dIset exceeds the Tset, the grounding fault of the pole is determined, so that erroneous judgment and misjudgment caused by other faults are avoided; meanwhile, the fault pole with the grounding fault can be detected; the accuracy for judging the direct-current monopolar grounding fault is improved; the fault can be detected through on-site measurement in a single convertor station; and the method is simple in program and easy to operate, and can quickly and accurately identify the direct current fault of the flexible direct-current transmission system.