Abstract: The high solid content of oil-based drilling fluid of ultrahigh density makes it inconvenient for its property control and maintenance. To solve this problem, the particle size distribution and microscopic morphology of micronized barite, micro manganese ore powder and common barite were analyzed indoors by means of laser particle size analyzer and scanning electron microscope (SEM) respectively. The properties of the oil-based drilling fluids of ultrahigh density weighted by the combination of micronized weighting agent and common barite at different proportions were investigated. And the weighting formula for the oil-based drilling fluids of ultrahigh density was controlled by changing the dosage of treating chemical. It is indicated that the oil-based drilling fluids of ultrahigh density which is weighted by the combination of micronized weighting agent and common barite at different proportions is better than that weighted by the common barite in terms of rheological property, electrical stability and wall building property. The optimal combination proportion between micronized barite and common barite is 5:5-6:4. When micro manganese ore powder and common barite are combined, the higher the proportion of micro manganese ore powder is, the better the property of the system is. Considering the cost of micronized barite and micro manganese ore powder, the oil-based drilling fluids of ultrahigh density is weighted indoors by setting the proportion between micronized barite and common barite as 3:7 and the proportion between micro manganese ore powder and common barite as 2:8. Based on this, the weighting formula is optimized by adjusting the dosage of organic clay and emulsifier and changing the internal phase. Thus, the oil-based drilling fluid of ultrahigh density with good properties is prepared.