Abstract:The water retention curves of some kinds of infiltrative rock masses in which grow a great deal of capillary have been applied in some researches. But at present,to get the water retention curve of fractured porous rock exactly by experiment is still very difficult. And the water retention curve can only be gained from mercury- injection pressure curve. But the accuracy of the water retention curve directly decides the accuracy of the computational parameters of the unsaturated rock. So how to get the water retention curve of the fractured porous rock accurately and efficiently is worthy of researching. With the rock microcosmic structure experiment,based on microcosmic structure of rock and capillary theory,a predicting method of water retention curve for fractured porous rock is discussed,and interstitial sandstone is put up as an example. Gray scale images of red sandstone are changed into digital images with high-speed image acquisition card,and then microstructure images of red sandstone are analyzed by means of Geo-image programme. Characteristic factors of porous sandstone are determined. They are porous rate,aperture number,aperture distribution,granularity distribution,mean area,mean perimeter and so on. The aperture distribution almost obeys Gamma distribution. A series of different suction values with corresponding water contents are determined for matric apertures and micro fractures,and water retention curves of matric apertures and micro fractures in sandstone are predicted. For the fractured porous sandstone,total porous rate equals aperture rate plus fractured rate so that synthesis water retention curve is obtained. The predicting water retention curve is compared with mercury-injection pressure curve. The result shows that the two curves can inosculate well. It can be seen that this method is an effective approach for getting the water retention curve of interstitial rock which is very convenient and practical.