There has been considerable controversy over the formation age of the Luohansi Formation in Qinling orogenic belt. Based on the LA- ICP- MS zircon U- Pb isotopic dating, the formation age and material source of the Luohansi Formation have been analyzed from tuff and epi- metamorphic detrital rocks, in the eastern part of West Qinling orogenic belt. The results show that the crystallization age of the tuff in Member a of the Luohansi Formation is 407±10 Ma. The captured zircon ages of phyllite detrital zircon in upper clastic member (Member c) range from 2658 to 420 Ma, mainly including the peak ages of 490~420 Ma and 997~639 Ma, followed by 1557~1318 Ma and 2484~1745 Ma, and a small amount of about 2650 Ma. Combined with the regional geological data, the primary formation age of the Luohansi Formation is defined as Early―Middle Devonian. The study also shows that the material source of the Luohansi Formation is diverse. The age group from 490~420 Ma comes from magmatic arc rocks formed by the Shangdan ocean subduction; the 997~639 Ma age group can be further subdivided into 843~639 Ma and 997~867 Ma, the former mainly comes from the South Qinling tectonic belt, the latter mainly comes from the North Qinling tectonic belt. The 2658~1318 Ma age group of the middle Proterozoic Neoarchean is related to magmatic activities in the North Qinling tectonic belt, North China plate and Yangtze plate. Comprehensive analysis shows that the South Qinling and North Qinling blocks did not ultimately collide into land during the Early- Middle Devonian, and the Shangdan ocean is a residual ocean basin.