Abstract: The classification methods of drilling fluid systems are reviewed and illustrated, e.g., dispersing medium classification, API classification and IADA classification. A new classification method based on drilling fluid density is presented, and five categories can be classified by this method, which covers drilling fluid of low density of less than 1.0 g/cm3, drilling fluid of ordinary density of less than 1.2 g/cm3, drilling fluid of ordinary weighted density of 1.2~1.6 g/cm3, drilling fluid of high density of 1.6~2.3 g/cm3, and drilling fluids of ultrahigh density of more than 2.3 g/cm3. This method is easy for drilling fluid selection and field using.