Abstract: Pre—Bogda mountain folded zone is one of the most complicate and grand tectonic unit at premountain area in China. Starting with anaiysis of the aerial colour infrared photography and combining with the survey of outcrops and use of the vast exploration data in recent years, the writers restudied the structure of pre-Bogda mountain area and correctly described this arched tectonic system which is com posed of large anticlinorium and synciinoria and imbricated faults. The writers also probed into the mechanism of formation of tectonic structure and pointed out that oniy many large reverse faults, rather than tectonic nappe, developed in here. The cardinaj cause of these particular violent structure formed in Yenshan movcmeni was the result of premountain depression without a consolidated basement. The pre—Bpgda mountain depression was formed by directly succeeded to the gradually evolution of late一Hcrcynian geosyncline, so it is a new type of pre—mountain depression.