The invention provides a slab width detection system based on a guide ruler control program set at the entrance of a sizing press. A slab width detection control program is added in the guide ruler control program, the slab width detection control program is respectively connected with a guide ruler alignment control program and a guide ruler position control program, simultaneously, the output end of the slab width detection control program is also connected and communicated with an operating picture via a basic automation network, and the operating picture is connected with an offset alarm system. In the invention, without any investment, via the guide ruler alignment control program and the additionally set slab width detection control program at the entrance of the sizing press, the width offset of the slab is quickly recorded, fed, calculated and displayed, and an alarm is given out in case of out of tolerance, so that the operator can find the problem timely and handle the problem, the width defect of finished products is avoided, waste products are reduced, and both the qualified rate and the rolling yield of band steel are improved.