A mathematical model that expresses the relationship between Near-infrared light intensity and automatic threshold for automatic kiwifruit surface defect detection was established. By applying different levels of Near-infrared light intensity to machine vision system, 268 images were collected. Then the images were processed with MATLAB using the method to detect kiwifruit defects based on Near-infrared light source .The obtained 268 sets of data on Automatic Threshold T0 and Manual Threshold T1were divided into 19 groups according to different aperture and light intensity. After processing data, a series of linear equations about the relationship between Near-infrared light intensity and Automatic Threshold T0, with function fitting coefficient of R2 > 95% was obtained. Finally, relationship between T0 and T1 was analyzed according to the effectiveness of image processing results and constant P was introduced to revise Automatic Threshold T0.Thus, a mathematical model needed to gain kiwifruit defects detection threshold, namely Model Threshold T, was established.