Abstract: For solving the problem of non-destructive testing method for crop nutrition, a non-contact photoelectric instrument was designed. The instrument could measure the crop nitrogen according to the visible-near infrared spectral characteristics of the nitrogen. Through measuring the two-way incident light and the reflected light, the instrument could measure the vegetation index accurately according to the automatic calibration techniques based on the least squares estimation. By using the signal processing method, the problem caused by the weak signal and the sun angle was solved. This instrument measured the vegetation index conveniently, and the results had a very good correlation with the values measured by spectrometer, and the coefficients of determination were 0.994 and 0.985, respectively. The wheat experiments indicated that instrument could be used for non-destructive measuring the crop nitrogen. The instrument had the characters of low power consumption, high sensitivity and rapid response time. The crop nutrient measured by the instrument could be used for variable rate fertilization.