Abstract: According to the temporal variation of vegetation index and the correlation between vegetation index and yield in different periods， the best time for soybean yield estimation in Northeast China will be established. The correlation between crop NDVI and crop yield obtained from late July to mid-August was the strongest， and the correlation between NDVI in different periods was very weak. When NDVI was used to predict soybean yield， the of NDVI late July and mid-August was used. There is a linear positive correlation between NDVI and soybean yield， and the correlation between NDVI and soybean yield is not very high， indicating that NDVI is not the only variable related to soybean yield.

Key words: soybean, vegetation index, NDVI, remote sensing, estimation of yield