Abstract: The precision agriculture has become a research hotspot of agricultural science nowadays. How to acquire information of the plants is one of the key points and problems in precision agricultural research. This paper identified the relationships between leaf area index and spectrum data by spectral analysis, thereby achieving fast and accurate LAI measurement. The measurement system for citrus LAI based on spectrum information was divided into two main parts. One was the data acquisition device, with FieldSpec-FR and computers as its key components to collect spectral data of leaves. Another was the inversion platform, with computers, ViewSpecPro, Excel, SPSS and other software as its core to calculate the LAI of citrus leaves. Researching the features of LAI can provide crop biomass status information for precision agriculture. And the exact total leaf area and the nominal LAI values of citrus leaves can be calculated after a scanning calibration experiment. The regression results showed that the system could meet the LAI test requirement.