Process Parameter of slub yarn determine style of its fabric. A new method of measuring process parameter of slub yarn was put forward to improve precision and efficient . The basic principal of parallel-plate capacitor was introduced in this paper. And sample yarns were tested with test system of slubby yarn developed by ourselves. Time sequence for slub length and slub space of slubby yarn can be gotten. And distribution of slub length , slub space and slub scaling factor can also be obtained. The agreement can be attained compared with those original setting. Some error was analyzed also. Consequently this testing system is effectively, and can be adopted in practice.