The invention discloses a system for controlling the rolling reduction of a metal plate straightening machine. The system comprises a weight detection unit, a rolling reduction adjustment unit and a controller, wherein the weight detection unit is used for detecting the weight of a metal plate to be straightened; the rolling reduction adjustment unit is used for adjusting the rolling reduction at an inlet and/or outlet of the straightening machine; the controller is used for generating a control signal according to the weight of the metal plate to be straightened so as to control the work of the rolling reduction adjustment unit according to the control signal. The control system is capable of adjusting the rolling reduction of the straightening machine according to the weight of the metal plate so as to enable the rolling reduction of the straightening machine to be more reasonable and enable the straightening machine to have the advantage of being good in straightening effect. The invention also discloses a method for controlling the rolling reduction of the metal plate straightening machine.