Abstract: in order to accuratels characterize the porosity vaiation of the less saturated coal reservoir during the development process a porosity model of the coalreservir was esablished based on thecitial desorptio pressure as the boundary and in consideration of the cal matix shinkage and fctivestress couping roleThus with the variation law of the porosity pressure the porosity during the mining process was analyzed. The study resuts showed that in the development prcess.theporosit varation would have four sages and thy would be the water drainage and prssure releasing stage the sow pressure releasing stage supplement rebound stage and rapid increasing sag .Those stages would be relevant to the gas and water output variaion stage in the producion process.The porosity variation degree wasmanly contrlled by the confguration relationship betwee the rebound pressure pit and the crtitaldesorpticn pressure. when the rebound pressure was closed fo the critical desopion pressure the matik shrinkage strength would be higher and the rebound capacityof the porosity would be sronger:Within the pressure released hoppe formed durng the producton proc.ss,each poit porosity variation could refect the pressurevariation of the coal reservir.could expand the rebound pressure surtace of theporosity in the prssure released and hopper and theporosity pressure suface and could be favorableto inprove the producion capacityof the coalbed methane well.