A new near infrared diffuse reflectance spectroscopy method is proposed to rapidly detect α-cellulose content of natural cellulose (plant fiber: cotton, wood) pulp in a new clean pulping process. One hundred forty two samples were collected and their α-cellulose content data were determined by standard method GB/T 9107-1999. The samples were homogenized by grinding pretreatment to improve spectroscopy measurement accuracy. Effective classification models were built by SIMCA, with the total correct identification. Using partial least squares (PLS) quantitative calibration, α-cellulose of the whole and separate cotton and wood pulp was established, with the correlation coefficients of 0.954, 0.911, 0.839, SEP, 0.024, 0.012 and 0.016, respectively .The repeatability results obtained by the new method are in agreement with the results from GB/T 9107-1999. The new method is feasible for determining α-cellulose content of natural cellulose (plant fiber: cotton, wood) in clean pulping process.