Abstract: This study used digital image acquisition via digital camera and analysis method in combination with rapid soil and plant N test to diagnose the state of N in wheat crops and recommend N fertilization modes in wheat fields. The relationship between color parameters and N nutrient indices was analyzed and the possibility of using digital image analysis method to diagnose the state of plant N evaluated. Then a standard method for digital image acquisition and application in plant N diagnosis was established. The best mode for diagnosing the state of plant N via digital image analysis method was also determined. The study further established a N diagnosis index system and an equation for recommending N application. The result indicated that digital image processing technique was usable in detecting the N state in winter wheat. The best angle between digital camera and wheat canopy in the field was 30°~60°. Significant correlations were noted among color parameters and N application rate, SPAD, plant nitrate content, plant total N content and nitrate contents in the 0~90 cm soil depth at wheat jointing and booting stages. Compared with other plant canopy color parameters, normalized greenness intensity (NGI), calculated as G/(R+G+B), was much better as it produced better correlations with plant total N, SPAD, nitrate content and soil nitrate content at jointing stage. While NGI was the best diagnosis index, jointing was the key diagnosis stage of the state of N in winter wheat crops.