The invention provides an automatic cutoff device for long steel wire bristles. A first motor used for driving roller driving is arranged inside a frame, a support frame used for driving roller installation is arranged on the upper surface of the frame, a bearing pedestal is arranged on the support frame, a guide groove is formed in one side of the bearing pedestal, a sliding rail matched with theguide groove is arranged on the inner side face of the support frame, a rotating handle is connected to the upper surface of the bearing pedestal through threads, and the rotating handle penetrates through the upper surface of the support frame; at least two driving rollers are arranged on the frame, the driving rollers are connected to the first motor through a belt device, a cutting device is arranged at one side of the frame, the cutting device comprises a second motor and a cutting tool arranged on a second motor shaft, material guide channel steel is arranged between the cutting device and the driving rollers, and drivers are arranged on the first motor and the second motor respectively. Accordingly, the steel wire bristles with the length meeting the requirements can be cut off automatically and rapidly, the operation strength is small, and the device is suitable for requirements of large-scale production.