The diagnosis technique of real-time lossless crop nutrition is the foundation and conditions for the precise and effective fertilization. Currently, the diagnosis of crop nutrition mainly relies on the routine chemical analysis of laboratory. Due to the complicated procedure, time-consuming, high cost and high professional technique requirement, it can hardly meet the need of precise variable fertilization technology. Spectrum technology is the technology of real-time and non-destructive testing, and its development has rapidly promoted in recent years. Therefore, it is widely adopted in many fields, such as Chinese medicine chemical analysis, on-line quality control, while less adopted in real-time diagnosis of field crop research and progressed slowly. In order to explore the diagnostic technology based on spectrum of soil and leaf nutrient in citrus orchard and provide evidence for the development of real-time, accurate, and non-destructive citrus nutrition, we have carried out real-time detection technology research in citrus orchard based on the visible-near infrared spectroscopy detection of the soil and leaf nutrient element contents.