Abstract: The main advantages of Casson Model are its strong suitability, its convenience in calculation, its high accuracy and its better capability of reflecting the rheologic laws of drilling fluid at different shear rates. The principal rheologic parameters in the model are closely related to the properties required by drilling fluid when it is in motion and at rest. Owing to the fact that the model was used in drilling fluid not long ago, there are no completely agreed opinions on it. Based on the articles published already, this paper gives diffcrent opinions on the basic meanimg and the calculating method of the rheologic parameters in Casson Model，and this is for reference only.Necessary descriptionsb are given of the ability of Casson Model to
carry cuttings and the degree of its shear thinning, and preliminary con-clasions are also arrived at