The device is composed of a power output device consisting of a testing wheel and a oil pump, a logic valve and an oil cylinder, and then connected with the self-aligning idlers bracket. It is normal operation when the belt moves in the middle and does not contact to the two testing wheels. If the belt runs to the left, the left power output device get started, the actuator will adjust the rectifying roller position according to the judgement of logical valve to generate a reverse thrust, and push the belt to the middle. Contrarily, if the belt runs to the right, the right power output device get started, the actuator will judge the rectifying roller position according to the judgement of the logical valve to generate a reverse thrust, push the belt to the middle. The idlers make the belt always running in the setting width range, avoid belt tearing, machine halt, material sprinkling due to the belt deviation, ensuring the normal running of the conveyor.