The cotton′s maturity and fineness are the key parameters to evaluate the cotton fibers quality. The indirect parameters measurement can not accurately evaluate the cotton fiber′s quality. A direct method based on the optoelectronic measurement of Fraunhofer diffraction was presented. The measurement results were compared between the microscope and the optoelectronic setup. The results show that two methods have the good correlation(R=0.956, and SD=0.332). The optoelectronic technique can assess the cotton fiber′s maturity in the lab.