The state of crop growth can be reflected truly from prominent morphological characteristics of leaf vegetables' veins. Meanwhile, computer image processing technology enables these features to be extracted, which provide a significant method to master the vegetables growing and allows the environment of plant growth cabinet to be controlled in intelligent way. This paper analyzes the histogram distribution of each channel firstly, and then combining the color channel combination operation with threshold segmentation to get the veins and blade profile images. After that, the blade edge is extracted through mathematical morphology. Lastly, this paper compares the absolute value between the profile images and the blade edge images to achieve precise segmentation and extraction of the veins. The results of experiments show that it is a better method to extract veins by means of the difference between mesophyll and the leaf color, allowing the manager to measure the crop growth and making it possible for intelligent monitoring used in plant growth cabinet.