The Palaeogene fractured saline muddy limestones are the important reservoir in the western Qaidam Basin. There are shear fracture, tensional fracture, sliding fracture and diagenesis fracture in the Palaeogene reservoirs. Fractures are generally filled with calcite, gypsum and glauberite. The validity of fracture is very important to reservoir evaluation. Effective fractures not only provide storage spaces and channels for fluid flow, but also determine enrichment rule and potential throughput of oil and gas. The validity of fracture is controlled by forming time, solution, uplift and denudation, abnormal fluid high pressure and distribution of gypsum and salt beds. Late fractures are more effective than early fractures. Solution can obviously increase the validity of filled fractures. Uplift and denudation from Quaternary and abnormal fluid high pressure can make filled fractures reopen and have good validity. Fractures distributed far away gypsum and salt beds have better validity. The effective fractures influenced by fault activity are not necessarily aligned with the maximum horizontal stress. Fractures parallel to main faults have the best validity.