The method to provide crop biomass status information for precision agriculture by researching the features of leaf area index(LAI) is presented in this paper. The relationship between LAI and spectrum can be identified by spectrum analysis, thereby achieving fast and accurate LAI measurement. The exact total leaf area and the nominal LAI values of citrus leaves can be calculated through a scanning calibration experiment. Analysis system for citrus LAI spectrum data is made up of a data acquisition platform with FieldSpec-FR and computers as its key components to collect spectrum data of leaves and an inversion platform with computers, ViewSpecPro, SPSS and other softwares as its core to calculate the LAI of citrus leaves. The fitting equation of LAI and spectrum information obtained is Y=1.5106X+0.7869 and the calculation error is ±0.0378%. The fitting equation of LAI and dry weight obtained is Y=0.0924X+3.1252 and the calculation error is ±0.3002%. The fitting equation of dry weight and wet weight obtained is Y=0.1807X-0.1133 and the maximum calculation error is 2.2713%.