Abstract: In this paper, the theoretic equations of the maturity ratio and other indexes of cotton fiber measured by airflow instrument were analyzed and it is found that the measurement of the maturity and linear density of cotton fiber by two-compressed airflow instrument is unreasonable. Furthermore, we theoretically modified the theoretic equation of airflow instrument. The results indicate that there are at least three independent variables in the theoretic equation of airflow instrument, including linear density, maturity ratio and mean outer equivalent diameter in the middle part of cotton fiber. In addition, the variety of cotton fiber can also affect the testing results of airflow instrument.