Monitoring the soil moisture variety in large-scale is very important to establishing the global water-cycle model

and to forecasting the weather and the floods.The spaceborne microwave radiometer is an efficient way to monitor the soil moisture variety in large-scale.When the radiometer observes the earth from the space

the absorbing and the scattering affection in the vegetation layer will attenuate the up-forward radiance from the soil

and this affection must be counted and reduced during the process of soil moisture retrieval.Much ancillary data are needed for computing the attenuating effects of the vegetation layer in the soil moisture retrieval model before

and the ancillary data are often unavailable.This paper proves that the Microwave Polarization Difference Index（MPDI） can be used to indicate the vegetation coverage conditions with the AMSR-E L2A brightness temperature data product.In this paper

we select the north-east and east part of China as the experiment area and the AMSR-E L2A brightness temperature and MODIS data as the sample data

and get the negative exponent relationship between MPDI and NDVI.Based on the relationship function and the knowledge about NDVI

MPDI has three threshold values that stand for High

Medium and Low vegetation coverage level.The threshold values can be used in the process of the soil moisture retrieval to judge the vegetation coverage level at the observed point and work out vegetation opacity.