Abstract: During the drilling process of horizontal wells in high temperature and high pressure gas reservoir in Yinggehai Basin, South China Sea, the rheological property of high density drilling fluids is difficult to control, the wearing degree of the highly-deviated well section casing is severe, the toque drag and drilling string are large, and the cementing quality of highly-deviated well section is hard to assure, etc. Based on this situation, to ensure the drilling safety and improve drilling efficiency, research work have been done on casing program design, oil-based drilling fluids technology resisting high temperature and density, the weighting technique of micrometre-scale barite, filter cake flushing technique of high density oil-based drilling fluids, the optimization of casing materials for high temperature and high pressure gas well containing CO₂, and the safe drilling technology of HTHP horizontal wells, etc., all of which have formed the key drilling techniques of HTHP horizontal wells in mid-deep strata of the Yinggehai Basin, South China Sea. The field application showed that the techniques could effectively ensure the safety and high-efficiency drilling, improve the cementing quality, and the blowout production capacity is 30% higher than expected. The key drilling techniques of horizontal well, which could provide technical support for the drilling of HTHP horizontal wells in mid-deep strata of the Yinggehai Basin, South China Sea, could also be applied in the wells of the same type under the same condition.