In order to access the growth information of crop fast, non-destructively and in real-time, a portable crop growth monitoring diagnostic instrument was developed based on spectral monitoring mechanism of crop growth indicators. The instrument included a multi-spectral sensor, the processor and affiliated institutions, which can access the growth indexes of crop, such as leaf nitrogen content, leaf nitrogen accumulation, leaf area index and leaf dry weight. The multi- spectral sensor contained two optical channels to collect the crop canopy spectral data at the wavebands of 720nm and 810nm. Optical filtering technique was used to improve the input signal to noise ratio of optical radiation. Based on crop canopy characteristics and operating environment in the field, the suitable structure parameters of optical channels were designed, ensuring that sensitivity and resolution effect of multi-spectral sensor. The processor was consisted of weak signal conditioning circuit, microcomputer and interface circuit. The processor was used to receive, display and store all the date from sensor, fuse the crop growth monitoring model and obtain the crop growth indicators. The calibration tests verified a good performance in terms of the static and dynamic characteristic, linear correlation coefficient was over 0.95, the hysteresis error of optical channels at 720nm was 3.35%, and at 810nm was 2.22%, with the smaller measurement error caused by the solar light irradiation changes, the average error of optical channels at 720nm was 1.43% and at 810nm was 1.38%. The rice experiments showed that the instrument has excellent performance on monitoring growth indexes of crop, the value of DVI had relationship with rice canopy leaf nitrogen, nitrogen accumulation, leaf area index and leaf dry weight, the correlation coefficient R² was  0.8596, 0.8393, 0.8795 and 0.883 respectively, and  average measurement error of which were 6.14%, 8.29%, 6.24%, 6.18% respectively. The structure of instrument has character as simple, highly integrated, cost-effective, convenient to carry and easy to operate in fields.