Abstract: Abstract: To solve the problem of real-time, fast and non-destructive testing method for crop growth information, a portable crop growth monitoring diagnostic instrument was developed. Multi-spectral sensor, the processor and affiliated institutions composed of the instrument, which can access the growth indexes of crop leaf nitrogen content, nitrogen accumulation, leaf area index and biomass information. The multi-spectral sensor contains 720 nm and 810 nm detection lens, and structurally is divided into up-sensor and down-sensor. The detection lens of sensor is composed of spectral filter and photo-detector, which makes lights path is simplicity and reliability is better. The processor includes sensor signal conditioning circuit and monitoring system. Signal conditioning circuit is made up pre-amplification circuit, filter circuit, adjustable gain circuit, adjustable sensitivity circuit and signal standardization circuit, which can abstract characteristic spectrum information. The monitoring system uses stc89c516 microprocessor as core, and supplies four working modes, such as measuring, monitoring, diagnose and reset, with man-machine interface. The apparatus was calibrated by testing seven types of standard reflectivity board and calibration equation was founded. The rice experiments showed that the value of DVI measured by the portable crop growth monitoring diagnostic instrument had relationship with rice canopy leaf nitrogen, nitrogen accumulation, leaf area index and leaf dry weight, the correlation coefficient R2 was 0.711, 0.8231, 0.7698, 0.7212 respectively. The structure has character as simple, highly integrated, cost-effective, convenient to carry and easy to operate in fields.