Abstract: The western Tarim Basin, located in the interior of the Asian continent and far away from the ocean, is a typical arid desert landscape. However, the western Tarim Basin was marine environment before 34 Ma. The famous “Shell Mountain” in Wuqia County provides the direct evidence of the transition from sea to land in the western Tarim Basin. Based on the detailed study on the strata and fossils of the “Shell Mountain”, the process of how the western Tarim Basin changed from sea to land are carried out. The “Shell Mountain” is composed of marine oyster fossils in the middle Eocene (~46 Ma), indicating that the western Tarim Basin was a gulf in the northeastern margin of the Neotethys Ocean, with warm climate, shallow water, and rich in marine organisms represented by oysters. As a result, a set of thick shell layers, namely, the Karatal Formation, is deposited. During the late Eocene to early Oligocene, the sea gradually retreated from the western Tarim Basin. With the continuous uplift of the Tianshan Mountains and Kunlun Mountains since the late Oligocene, the previously buried shell layers are re-exposed, forming the present Shell Mountain.