Abstract: The main reservoir in Tahe Oilfield of North Tarim can be classified as buried hill reservoir, and it is located in the carbonate rock formation in the Ordovician system.Since the success of solids free polymer drilling fluid system in the under-balanced drilling of well S66, the system has been used in the under-balanced drilling of 16 wells, including some vertical wells and horizontal wells.Field application shows that the system has stable performances and good cuttings transportation, and can fully meet the requirements of under-balanced drilling in the Ordovician system in north Tarim.The performances and field application of the solids free polymer drilling fluid system are described.