In this research,near infrared spectroscopy was used to detect adulterated percent of camellia oil adulterated with soybean oil quantificationally at different optical lengths,and the effect of optical length on detection accuracy of adulterated percent was investigated.Soybean oil was put into camellia oil according to different mass fraction,the adulterated mass fraction was ranged from 1% to 50%.Transmission spectra of samples were acquired by a Quality Specspectrometer at different optical lengths(1,2,4,10 mm),and effect of optical length on detection accuracy of adulterated percent was analyzed by comparing quantitative prediction models that developed at different calibration methods,pretreatment methods and wavelength range.The results indicate that the performance of quantitative prediction model of adulterated percent is improved as the optical length is increasing from 1 to 4 mm,while the performance of quantitative prediction model of adulterated percent is deteriorated as the optical length is increasing from 4 to 10 mm.4 mm is a better optical length for camellia oil adulteration.The coefficients of determination of prediction(R2P) and root mean square error of prediction(RMSEP) in quantitative prediction models of adulterated percent for optical lengths of 1,2,4,10 mm are 0.923,0.977,0.989,0.962 and 4.58%,2.54%,1.72%,3.20%,respectively.