Abstract: High degree of accuracy slub yarn can be produced by EJM128K model ring bobbin with servo-actuator of PLC control and mechanics change.In this paper，With transfer function of the control system was analysed ,this close loop control system possess stability ,precision and rapidity at parameter of slub length.The slub yarns were tested with USTER® TESTER5, and the distribution curve of slub length and slub space can be attained.Therefore,slub length deflection was influenced on slub length set ,slub yarn twist factor and fiber nature etc. This result can control effectively slub length and produce high-class slub yarn.