The invention provides a dynamic reactive reserved optimizing method for improving the long-term voltage stabilization in an AC/DC (Alternating Current/Direct Current) power grid. The method comprises the steps below: determining a key failure integration for affecting the long-term voltage stabilization in the AC/DC power grid; adjusting reactive power output of dynamic reactive compensation equipment, and calculating the flexibility of the dynamic reactive compensation equipment; ordering m dynamic reactive compensation equipments, and calculating the weighting coefficients of the dynamic reactive compensation equipments; calculating the reserved capacity of the dynamic reactive compensation equipments, and building and resolving a dynamic reactive reserved optimization model. According to the method provided by the invention, the auxiliary decision support can be provided to improve the long-term voltage stabilization level in a multi DC droppoint power grid; furthermore, the dynamic reactive reserved optimizing method has significance on the improvement on the long-term voltage stabilization allowance in the AC/DC power grid, the construction of the smooth power transmission channels between a transmission end and a receiving end, the improvement on the delivery capacity of a DC power transmission channel, and the improvement on the economy and power quality of the operation of the power grid.