Compared with the existing ontology construction methods, the approach of building oilfield domain ontology based on Thesaurus is proposed, which can alleviate the amount of work involved in building ontology, and solve a series of problems in the process of transformation from the thesauri to the ontology, such as the selecting of some uncertainty concept, the representation of some uncertainty relationships among words in thesaurus and the refinement of those relationships. This paper proposes a series of principles which describe the refinement of “USE、UF、NT、BT、TT、RT” five relationships in thesaurus, and presents how to use OWL language to describe the thesaurus and several relationships among words, so as to realize the OWL description of the Thesaurus.