The invention discloses a reactive power controller and a reactive power control method and system of a flexible excitation system, and belongs to the field of generator excitation control. The methodcomprises the following steps: n typical oscillation modes and wide-area measurement signals with the highest observability are obtained to be respectively used as optimal wide-area measurement signals of n channels; after filtering out the oscillation signal in the region and compensating the phase of each optimal wide-area measurement signal, the optimal wide-area measurement signals are processed according to the projective control theory so as to obtain the first reactive power command value corresponding to each channel; the wide-area measurement signals of the power system are obtainedin real time, and the current oscillation mode is identified when the system oscillates; and after calculating the weight of each channel according to the membership degree between the current oscillation mode and each typical oscillation mode, the first reactive power command values corresponding to each channel are respectively weighted, limited and summed so as to obtain the second reactive power command value for suppressing the current oscillation. The reactive power control of the flexible excitation system can be realized and the ability of suppressing the inter-regional oscillation canbe improved.