Abstract: The characteristics of DMSP SSM/I's channels were introduced.The absorbing and scattering effects of the atmospheric particles(especially cloud droplets,rain drops)were analyzed.The physical reason of typhoon patterns representation on microwave image was revealed.An index of precipitation was then induced as such:Sym metrically inverse channel 85 GHz's absorption segment and put it into the ext ension of t he scat tering seg ment.The precipitation index was formed as the average of three elements:uniformed 19GHz,uniformed 37GHz,and inversely uniformed 85GHz.Three channel composed precipitation index overcame some shortcomings of the single raw channel,e.g.,:37GHz is not sensitive to large rain drops,85GHz is not sensitive to medium-size rain drops.It was shown that the image of precipitation index could reveal typhoon's spiral structure more clearly.