The invention discloses a reactive power control optimization method based on a flexible DC system, which comprises the following steps of: setting an AC voltage low threshold value, a high thresholdvalue, a reactive power maximum value and a minimum value; When the difference between the actual AC voltage and the AC voltage is less than zero, the current power value is taken as the initial valueof the integrator. When the difference is greater than zero, the maximum reactive power is obtained when the integrator reaches the full bias state, and the output of the integrator is regarded as the upper limit of the reactive power. Taking the difference between the actual AC voltage and the AC voltage high threshold as the input of the integration link; When the difference value is greater than zero, the current power value is taken as the initial value of the integrating link, and when the difference value is greater than zero, the current power value is taken as the initial value of theintegrating link. When the difference is less than zero, the reactive power is the minimum value when the integrator reaches the full bias state, and the output of the integrator serves as the lowerlimit of the reactive power. The stable operation of AC voltage can be ensured even in the mode of constant reactive power control of flexible DC system.