The width of the tobacco fiber is an important indicator of the quality of the cigarette. In order to be able to control the quality of tobacco fiber width in real-time, this paper designs a real-time online detection method of tobacco fiber width. The SPP and PANet structure were fused in YOLOv4, and the three output scales were changed into one output scale making the improved YOLOV4 algorithm simpler, and the calculation speed increased nearly 35% in the same case of performance. The width calculation method was the MCS (Moving Center Search) center mobile search method, which could automatically search for both sides of the tobacco fiber, and more accurately calculate the width with the calculation accuracy of 0.2 mm. The MCS tobacco fiber width calculation method was combined with the improved YOLOv4 model of this paper, and real-time online tobacco fiber width detection was performed to control the quality of tobacco fiber in real-time and improve production efficiency.