Abstract: The hydrothermal experiments of volcanic clastic rocks chosen from the Nanbeier Sag of the Tamtsag Basin and formation water were researched under different temperatures in a closed container.The results show that quartz can be dissolved by alkaline formation water and the dissolution intensity will be enhanced with the increasing of temperature;on the contrary,the dissolution degree of feldspar and calcite is very low.Meanwhile,by scanning electronic microscope (SEM),phillipsite is found on the surface of rhyolitic tuff sample (at the temperature of 100,120,140 and 160℃) and sedimentary tuff sample (at the temperature of 120 and 180℃);calcite is found on the surface of sedimentary tuff at 100℃.Considering the good preservation of quartz and strong dissolution of feldspar and carbonate,it is proved that the formation water contributes few to the secondary poro-sity of the Nanbeier Sag of the Tamtsag Basin.