During the period of Late Carboniferous and Early Permian, the sandstone with abundant volcanic dust fillings was mainly deposited in a peat marsh environment, which constituted a favorable reservoir of coal-formed gas in Ordos Basin. Studies show that the original fillings of sandstone are mainly superficially hydrolyzed volcanic dust, which is a superfine mixture of aluminosilicate and silicate. It is difficult for volcanic dust to form zeolite during hydrolyzing under the condition of acidic syngenetic water in a peat marsh environment. The tuff fillings of sandstone are susceptible to the property and activity of the formation water, and have mainly experienced synaeresis, dissolution, alteration, carbonate metasomatism and recrystallization in a burial environment. The evolution of tuff fillings of sandstone can improve the quality of reservoir locally, but enhance the heterogeneity of reservoir regionally.