Ordovician karst-cave reservoirs develop in Tahe Oil Field, Tarim Basin. Sandstones filling in these Ordovician paleocaves are excellent oil reservoirs. Through the observation and analysis of cores and logging data from some wells in Tahe Block 7, especially for those of Well T615 as the typical research object, and associated with the investigation into the contrastable Ordovician outcrops far in the northwest of the basin, the development characteristics and genetic mechanism of filling sandstone in Ordovician paleocaves are ascertained in the oil field. The analysis result indicates that the sandstone filling in paleocaves was the no-barriering coast sand sediments that filled into the Ordovician karst caves along underground rivers the during Late Devonian Donghetang sedimentation. The filling sandstone consists of sublitharenite and quartzose sandstone. Contributing to good quartz sorting, high porosity and permeability, and developing primary intergranular pores and solution pores, the filling sandstone exhibits an excellent oil reservoir property. It is suggested that in the future exploration and development, it is necessary to carry out the researches including the sedimentary boundary of Late Devonian Donghetang filling sandstone in paleocaves, the spacial distribution of Ordovician karst caves and pre-Carboniferous micropaleogeomorphological feature.