The present invention provides a series compensation device and a capacity optimization method thereof. The device comprises a coupling transformer and a current converter, a primary side winding of the coupling transformer is in series access in a to-be-compensated line, a secondary side winding is in parallel connection with the current converter, and a plurality of tapping points is arranged on the primary side winding and/or the secondary side winding separately to change the voltage variable ratio of the coupling transformer. The method comprises the steps of obtaining an equivalent impedance value when the series compensation device carries out the inductive compensation, and adjusting the voltage variable ratio of the coupling transformer according to the equivalent impedance value. Compared with the prior art, according to the series compensation device and the capacity optimization method thereof, and by adjusting the voltage variable ratio of the coupling transformer dynamically, the capacity of the current converter always maintains at a constant state, thereby saving the capacity of the current converter, and further reducing the manufacturing cost of the series compensation device.