Abstract: The establishment of the detectible model of the cotton fiber mass density plays an important role in detecting the cotton flow for the pipeline of the online measurement through the pneumatic transportation. In this paper, the basic law of the light absorption was managed to study the attenuation of the cotton fibers for the light of the different wavelength. The results showed that changes of breed, moisture and ambient temperature had no significant relationship with the test results, when the near-infrared of the wavelength of 940 nm was selected as a light source. Therefore the near-infrared of the wavelength of 940 nm was selected as the emitter and the mass density measurement system of the cotton fiber was built. The decay rate of the light intensity of the cotton fiber was measured, the corresponding region of the actual measurement of cotton fiber quality was combined and regression analysis was used to establish the detectible model of the cotton fiber mass density. The validation results of the model demonstrated that the average error of the measurement results reached 6.1%.This research lays a foundation for on line monitoring of cotton flow rate in cotton pipe and developing sensors of cotton flow rate.