The invention relates to an optimum design method for an extra-high voltage multi-level flexible direct-current power transmission operation interval, and belongs to the field of high-voltage direct-current power transmission converter control strategies. The method comprises the following steps: obtaining upper and lower bridge arm currents of a converter through current transformer measurement, and averaging to obtain a current error generated by bridge arm alternating ring current; integrating the current error to obtain ring current voltage drop generated by the bridge arm alternating ring current; calculating direct-current voltage, internal phase electromotive force of the converter and the ring current voltage drop to obtain upper and lower bridge arm voltage reference values, and participating modulation. The method has the advantages that internal components of the converter do not need to be modified, and system parameters do not need to be changed, so that the original performances of the converter are guaranteed; the power operation interval of the converter is optimized on the basis of original equipment, and the original construction occupied area and relatively low investment cost are guaranteed. The power operation interval of the converter is optimized, and simultaneously the capacitor voltage fluctuation of sub-modules is reduced, so that the bridge arm ring current is inhibited.