The wavelengths of iodine laser are in an atmospheric micro-window. The absorption by water vapor is its main absorption besides aerosol in this region. The absorption coefficients and transmittance of several model atmospheres are calculated. In China, from south to north, from summer to winter, the water vapor density is decreasing, and the iodine laser beam absorption rate by water vapor is decreasing as well. It is better that an iodine laser beam transmits vertically upwards than horizontal flatly on the sea level. The absorption of an iodine laser beam with single main line is less than that with multi-lines.