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Based on time series Landsat8 images of atmospheric collected in 2015,，the change rules of normalized vegetation index (NDVI) and enhanced vegetation index (EVI) with the increase of vegetation coverage were studied. Their monitoring vegetation cover differences were analyzed quantitatively. The differences of NDVI and EVI distribution frequency curves and the differences of NDVI and EVI time series curves were analyzed. The results showed that NDVI and EVI were increased most rapidly when vegetation appeared on the surface, and with the increase of surface vegetation coverage, the increase rate of NDVI and EVI slowed a descending trend. NDVI with low vegetation coverage was increased faster than EVI, NDVI with medium vegetation coverage was close to EVI, and NDVI with high vegetation coverage was increased lower than EVI. Under different vegetation coverages, NDVI value was always greater than EVI value. NDVI and EVI distribution frequency curves can describe the number of pixels of different vegetation coverage and the change with time. NDVI and EVI time series curves can clearly reflect the growth change rule of a crop and the growth difference of different crops in the same period. The following conclusions were drawn: in the early stage of crops growth or under low vegetation coverage, NDVI and EVI overestimated vegetation coverage, and estimated value of NDVI was slightly higher than that of EVI’s. Their ability to describe vegetation was similar in the middle growth stage or in the middle vegetation coverage. EVI was more sensitive than NDVI to monitor crops growth during crops growth peak or under high vegetation coverage. To sum up, NDVI and EVI may be reasonably selected according to the change characteristics of vegetation coverage during crops growth period, and NDVI and EVI may also be used to supplement each other.