The invention belongs to metal plates bent linearly, and particularly discloses a brake device of a plate rolling machine. The brake device is characterized in that a rack, a stator, rotors, a current condenser and an electromagnetic brake used for stopping an upper roll are included; the rotors are fixed in the hollow upper roll; the stator is fixed to the rack and located in the rotors; and the rotors and the stator are electrically connected with the current condenser. Under the normal condition, when the upper roll rotates, the rotors are driven to rotate; and due to the fact that the stator is fixed to the rack, magnetic induction line cutting is generated between the stator and the rotors, and electric energy is generated and stored in the current condenser. When the plate rolling roundness does not meet the requirement, a power source is stopped, meanwhile, electric energy in the current condenser is used for starting the electromagnetic brake, and therefore the upper roll is stopped. According to the scheme, electric energy is generated when the upper roll rotates, collected through the current condenser and used for the electromagnetic brake, and therefore the upper roll can be stopped without additional electric power.