Abstract: The deep-seated fractured reservoirs in Bozhong 19-6 Oilfield are faced with high temperature, high risk of lost circulation, and other difficulties. To solve these problems, a high-temperature clay free drilling fluid system with the property of reservoir protection was developed by using high-temperature pattern adjusting agent, high-temperature colloid protecting additive, polyamine inhibitor and potassium formate weighting agent. Laboratory evaluation results show that the self-made key treating chemicals (i.e., high-temperature colloid protecting additive and high-temperature pattern adjusting agent) can be resistant to the temperature of 200 ℃, and they have the property of automatic gel breaking. The interfacial tension between the aged filtrate and the kerosene is 2.61 mN/m, indicating good inhibition. The permeability recovery under the condition of oil reservoir is over 90%, which indicates that the reservoir protection effect is good. This drilling fluid system has been successfully applied in 8 wildcat wells of Bozhong 19-6 Oilfield, among which Well Bozhong 19-6-7 has vertical depth of 5 508 m and bottom hole temperature of 190 ℃. In conclusion, this drilling system is applicable to the exploration and development of Archean high-temperature deep wells.