Abstract: The Advanced Microwave Sounding Unit-A (AMSU-A) is a new advanced generation microwave-sounding unit, and has great potential in land characteristic analysis, especially on flood detection and monitoring. The wetness index analysis method of flood detection and monitoring by using AMSU-A microwave data was tested over Boyang and Tongting lake areas in China during the 1998 summer. To accomplish this, the characteristics of microwave radiance transfer about AMSU-A channels are analyzed first and the result indicates that the window channels of AMUS-A can give more information about the surface flooding. Then a set of wetness indexed by combining three channels out of four AMSU-A window channels (ch1, ch2, ch3, & ch15) is developed. Thereafter the best wetness index which is sensitive to the land surface wetness changes is chosen to conduct the detection and monitoring, and the flooding area is classified into four situations: flooding, mudding, over wet, and back ground dry area. In the Cal/Val Test, and the data from China L-SAR aboard on the plane and Canadian Radar-SAR aboard on the Radarsat are used, the Cal/Val results is up to 73%. The future work will focus on the possible improvements in the algorithm and extend testing of the algorithm to other regions.