The invention relates to the technical field of a power grid and discloses a control device and method for improving positive damping of subsynchronous oscillation. The device comprises a velometer, a controller and a subsynchronous oscillation suppressor, wherein the velometer is connected with a power generation set and used for detecting a rotational speed signal of the power generation set and comparing the detected rotational speed signal with a standard rotational speed signal to acquire a rotational speed deviation signal of the power generation set; the controller is connected with the velometer and used for acquiring the rotational speed deviation signal and calculating a super-synchronous current instruction and a control pulse required for driving the subsynchronous oscillation suppressor according to the rotational speed deviation signal; and the subsynchronous oscillation suppressor is used for giving out a corresponding super-synchronous current based on the super-synchronous current instruction given out from the controller under the driving of the control pulse and injecting the super-synchronous current into the power generation set and the power grid so as to form positive damping electromagnetic torque in the power generation set for suppressing synchronous oscillation. By the control device, the problem that power grid flickering is easy to generate in the synchronous oscillation suppression is solved.