1. Since the Cenozoic Era the development of the Tianshan Mountains has gone through two periods of the formation of peneplain under denudation and planation in the Early Tertiary and the rise of high mountains corresponding to block faulting by a big margin from the Neog-ene to the Quaternary. Therefore, the evolution of natural environment is controlled.2. In Early Tertiary, replacement environment of sea and land sedimentation occurred in the southern piedmont of Tianshan Mountains, which was in contact with the Mediterranean Sea in some times; and in the northern piedmont of Tianshan Mountains there was terrestrial environment of river and lake deposits. Since the Late Tertiary, sedimentary environment has tended uniformity with slight difference in both mountain pieldmonts.3. Tianshan region belongs to a part of arid land between the zone of deciduous broad leaf trees of the northern temperate zone and the zone of evergreen broad leaf trees of the southern subtropical zone. Since the Tertiary, it can be divided into two relatively humid periods (the Palaeocene-Eocene and the Oligocene-Miocene) and three drought periods (the Early-Mid-Oligocene, Pliocene and Quaternary). The Quaternary period featured also some relatively humid and drought pulsation.