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MPI (microwave polarization index) method can use different frequencies at vertical polarization to retrieve soil moisture from TMI (tropical microwave imager) data,which is mainly suitable for bare soil.This paper makes an improvement for MPI method which makes it suitable for surface covered by vegetation.The MPI by using single frequency at different polarizations is used to discriminate the bare soil and vegetation which overcomes the difficulty in previous algorithms by using optical remote sensing data,and then the revision is made according to the different land surface types.The validation by using ground measurement data indicates that revision for different land surface types can improve the retrieval accuracy.The average error is about 24.5％ by using the ground truth data obtained from ground observation stations,and the retrieval error is about 13.7％ after making a revision by using ground measurement data from local observation stations for different surface types.The improved MPI method and precipitation are used to analyze the drought in Southwest China,and the analysis indicates the soil moisture retrieved by improved MPI method can be used to monitor the drought.