Abstract: Target spray is a focus in the field of agricultural information technology. The level of target spray varied with different target scale. It is a difficulty that taking a region of disease in a plant or one leaf as spray target. In this study, a mobile spray robotic system in greenhouse was developed, which sprayed the cucumber leaves precisely in a square of 0.2 m×0.2 m. The system was mainly composed of a mobile platform, manipulator, disease diagnosis system and variable nozzles. The mobile platform was installed in an elevated guide way, and it could stop according to the cucumber ridge location. The four degrees of freedom Cartesian coordinate manipulator was installed under the platform, which combined with spray bar to work in a rectangle of 1.2 m×1.2 m. Diagnosis of diseases was based on digital image analysis. A binocular vision system was constructed, which captured images in a region of 1.2 m×1.2 m, and then divided the images into units of 0.2 m×0.2 m. The level of disease was detected by analyzing images. Six spray nozzles were distributed uniformly on a 1.2 m long spray bar, and every nozzle corresponds to a region unit. According to the level of disease, spray time of nozzle was changed to realize variable spray.