Dual-spectra ratio method is generally considered superior to single-spectra approach in the most previous studies for water depth inversion by ETM image. In this paper, the correlation between the ETM's spectral and the water depth is analyzed. TM2 gives the largest absolute correlation index in all the single-spectra and dual-spectral ratio. Then some models using linear, logarithmic, power exponential and exponential equations have been made for water depth inversion. The exponential model based on TM2 is proved to be the best one, and it brings error of 24.6%.