Abstract: The basic principles and detecting flow in fruit internal quality detecting process were explained by the near-infrared spectroscopy. The three measuring methods, i.e. the on-line reflex, the partially shaded light transmission and the fully shaded light transmission, were analyzed and compared. Oranges and apples served as detected objects, the experiments on on-line sugar degree, acidity, internal brown change were undertaken by a fully shaded light transmission detecting device. Also, it provided a recession analyses based on the results of conventional analysis. The results showed that the correlation coefficiect between the actual value and detected value on-line is over 0.95,and the correlation coefficiect of acidity is over 0.85. The method of fully shaded light transmission can detect internal defect and quaily of fruit on-line.