Based on laser diameter-measuring technology, the measuring method on the cross-section diameter in fiber bundles and its wavelength of short-term irregularity is discussed. A contactless on-line measuring system and experimental scheme, including the selection of laser sensor and experimental materials, are plotted. The influence of feeding speed on the diameter, the unevenness and the mininum wavelength of the yarn and the results difference result from various methods are analysed experimentally. The results show that the system is feasible and has good reproducibility of the measured results. Moreover, the new system can extract the shorter wavelength feature on yarn short-term irregularity and the data analysis process can be efficiently automated.