The invention discloses a method and system for calculating the fault measure quantity of a converter transformer in a flexible DC power grid safety control system. The method comprises the steps of when the failure of any converter transformer in two pole layers of a flexible DC ring network bipolar structure leads to a locking fault of the corresponding converter, the pole layer corresponding tothe converter is a fault pole layer, and the other pole layer is a non-fault pole layer; acquiring the unbalanced power of the fault pole layer generated by the locking operation; acquiring the bandtransfer power of the non-fault layer; calculating the tripping quantity; and tripping terminal unit according to the tripping quantity. According to the method and system, the tripping measure quantity for tripping the terminal unit is confirmed through calculating the unbalanced power generated after the locking operation and the available band transfer power, and the band transfer power under various conditions is considered at the same time, thereby ensuring that the safety control system of the flexible DC power grid can calculate the correct fault measure quantity when the converter transformer breaks down, and being capable of effectively guaranteeing the safe and stable operation of the flexible DC power grid.