The invention provides a device for suppressing active overvoltage, a composite bypass switch, and a control method and system. The device for suppressing active overvoltage includes three single-phase bodies connected in parallel, each body includes an iron core formed by four iron core columns, two exciting windings reversely wound on the two middle iron core columns, and line side windings wound at outer sides of the two exciting windings. The device for suppressing active overvoltage is connected with the composite bypass switch in parallel, the composite bypass switch includes a bidirectional thyristor, a mechanical switch and an electric reactor, and a series branch formed by the electric reactor and the mechanical switch is connected in parallel with the bidirectional thyristor and each single-phase body in the device for suppressing active overvoltage. The device for suppressing active overvoltage eliminates self-excitation of a generator, and can be automatically adjusted to appropriate reactive compensation capacity, thereby eliminating a condition and phenomenon of generation of self-excitation, and creating conditions for direct access of a large unit to a power grid.