Abstract: Based on the clay feature of illite-momtemorillonite mixture dominately and gypsum intercalation in the formation of southern Huabei oil field, the MMH drilling fluid system was optimized in application. The purpose of addition of MMH positive gel is to enhance the rheology and inhibition of the drilling fluid. It needs to add filtrate reducer to control the filtration. But the addition of filtrate reducer will affect upon the positive performance of positive gel. To solve this problem, a number of lab testing and field trials on 12 wells have been conducted in three years. A MMH drilling fluid system was developed,which consists of MMH as main additive, BXJ as filtrate reducer and NPAN as viscosity breaker. For normal control of formation, the addition of MMH is 0. 2~0. 3 kg/m, that of BXJ 0. 4% and a little NPAN as required for viscosity reduction. This system exhibites unique rheology and strong sand carrying capacity and can increase ROP significantly and improve formation protection.