Sandstone reservoirs with a depth of over 6000m of the Lower Cretaceous Bashijiqike Formation in the Kuqa foreland thrust belt, Tarim Basin in NW China provide a good example for studying multiple-stage diagenetic fluid activities and effects of fractures on reservoir quality in tight sandstone. Three types of fractures can be identified in the sandstone: fully sealed fractures, partly filled fractures and minimally filled fractures. Fracture fillings are mainly composed of ferruginous dolomite, dolomite, calcite, quartz and gypsum. The ferruginous dolomite is mainly accumulated in the fully sealed fractures. The dolomite primarily occurs in the partly filled fractures and fully sealed fractures. The calcite and gypsum frequently occur in the fully sealed fractures, partly filled fractures and minimally filled fractures. The quartz is distributed at inner edges of the partly filled fractures. Contents of Co, Ni and V in the ferruginous dolomite are generally higher than that in the dolomite, calcite and gypsum, and contents of REE and LREE gradually decrease in turn from the ferruginous dolomite to the dolomite, and to the calcite. The δ¹³C of the ferruginous dolomite, dolomite and calcite ranges from 3.78‰ to -1.49‰ with an average value of -2.82‰, and their δ¹⁸O ranges from -16.43‰ to -12.3‰ with an average value of -14.62‰, respectively. The fully sealed fractures, ferruginous dolomite, partly filled fractures, quartz, dolomite, minimally filled fractures, calcite and gypsum successively occur from early to late stage. Data of δ¹³C and δ¹⁸O of the carbonate minerals in the fractures indicate that these minerals are formed in freshwater which is oxidizing with low salinities, and average formation temperature of the ferruginous dolomite, dolomite, and calcite is 89°C, 109°C and 123°C, respectively. Formation of these carbonate minerals in the fractures is primarily related to hydrothermal fluids originated from pyrolysis of organic matter. The fully sealed fractures serve as a barrier for fluid flow whereas the partly filled fractures and minimally filled fractures both provide channels for hydrocarbon migration and accumulation.