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The leaf area index (LAI) is an important parameter to characterize the canopy evapotranspiration of regional vegetation. For studying the ecological water requirement of vegetation in the groundwater-fluctuating belt in lower reaches of Heihe River, northwest China, a maximum growing season LAI was estimated by the regression relation between the measured value (from LAI-2000) and the estimated value of some spectral vegetation indexes, such as Ratio Vegetation Index (RVI), Difference Vegetation Index (DVI), Green Vegetation Index (GVI), Normalized Difference Vegetation Index (NDVI), Soil Adjusted Vegetation Index (SAVI) and Modified Soil Adjusted Vegetation Index (MSAVI) which were from the image of Landsat TM. The conclusions show that the relationship model between GVI and LAI was the optimal model to simulate the spatial distribution of LAI with correlation coefficient 0.66 in the study area. And the LAI value is changing from 0.004 to 3.753 in this region.