Abstract: Nitric acid concentration affects and even determines the distribution ratio of uranium and plutonium in reprocessing feed solutions, so, in the reprocessing process control analysis, the determination of the free acid plays an important role. Traditional laboratory analysis of the free aicd needs greater sampling, higher costing, and is time consuming. Hence, development of a new free acid fast analytical method has important significance for the reprocessing process control analysis. In this work, an NIR fiber-optic spectrophotometer designed and constructed in our lab has been used to determine total nitric acid concentration in samples from nuclear fuel reprocessing. Based on partial least squares(PLS) numerical analysis methods, calibration models are built from simulated and real -aqueous solutions with known acid contents and used to predict concentration of nitric acid. The acid detection limit applied in acid solutions by the established calibration model is 0.06 mol/L, and the detection limit for acid in mass plutonium is 0.12 mol/L, and the relative deviation is less than 10%, and the s_r is less than 5%. The NIR system can provide fast, accurate data with relatively small sample preparation. The PLS calibration results of the free acid concentr ation show good agreement with that of sodium oxalate fixed pH value titration.