The invention provides a flexible DC transmission power optimization method. The method comprises steps that on the condition that an electric power system under the condition of small disturbance or a single fault can stay in a stable state, flexible DC transmission power is adjusted, and a flexible DC transmission power scope is determined; the optimal flexible DC transmission power in the flexible DC transmission power scope is determined. The method is advantaged in that the flexible DC transmission power can be optimized, optimal distribution of power supply partition flows can be realized, power grid stability is improved, analysis and calculation are simple, and wide adaptability is realized.