Abstract：With its cloud-penetrating capability, the microwave radiometer can provide all-weather information of ground surface, except for rainy areas. WindSat, the first polarimetric microwave radiometer in the world, can measure all of the four Stokes parameters. The third and fourth Stokes parameters, which have never been measured globally and for a long time, can be used to study Sanjiangyuan region. The types of ground, such as water, vegetation, hungriness and so on, can be easily distinquished according to all of the four Stokes parameters. Small lakes and swamps may be mistaken as bare soil or hungriness by using only vertical and horizontal polarimetric brightness temperature TV and TH. So, the identification with four parameters can eliminate errors with respect to soil moisture. In this respect, the microwave radiometer is superior in studying Sanjiangyuan, where there are various types of ground. The trends of isolines of TV, TV－TH, MPDI (MPDI＝(TV－TH)/(TV+TH)) and the third Stokes parameters T3 are generally southwest-northeast for forest of conifer, plain of meadow, droughty plain, hungriness distributed from southeast to northwest. The microwave radiation changes seasonally along with melting and freezing of ice and snow, growing and decaying of vegetation. The differences between TV and T3 of forward looking and back looking show that the microwave radiation of Sanjiangyuan region is anisotropic. The anisotropy of snow and ice and lake is stronger than that of vegetation and hungriness.