In order to provide scientific basis for prospecting and mining coal - bed methane, it is necessary to analyze the instan- taneous pressure on bottom of coal - bed methane well. According to the characteristics of geology and considering vertical well and infinite coal bed, a mathematical model of coal - bed methane well in dehydration phase has been set up with certain assumption. The method and steps for analyzing the instantaneous pressure on well bottom are given, so that the relevant parameters of coal - bed and well can be achieved, which provides theoretical basis for the exploitation of coal - bed methane.