D7H well of Dongfang 13-1 of gas field in Western South China sea is the first offshore high temperature and high pressure slim horizontal well in China, its reservoir physical property is mainly of medium to low permeability, the drainage passage is easy to be blocked due to the use of the drilling fluid with high solid content, which causes reservoir damage; and being restricted by offshore completion method and up to 10 days retention time of drilling fluid in borehole, the extent of reservoir damage will inevitably be exacerbate. In view of this, a lot of indoor optimization evaluations were carried out, oil-based drilling fluid suitable for reservoir protection in offshore high temperature and high pressure slim horizontal well drilling and completion has been optimized. The pollution mechanism of high density drilling fluid is analyzed, the relationship among flow back pressure difference, flow back time and core permeability recovery results are studied and the measures to improve reservoir protection effect by flow back pressure difference increasing and flow back time prolonging are put forward. The experiment results show that after the core is contaminated by high density drilling fluid under high temperature, the initial permeability recovery value is about 85%, when the polluting time is up to 15 days, the permeability recovery value plunged to 30%, which shows that the longer the residence time of the drilling fluid in the reservoir, the more serious the pollution is; in the same displacement condition; along with the time prolonging, the permeability recovery value increases and becomes stable; the pressure difference increasing in flowback is conducive to increasing permeability recovery value, when flowback pressure difference reaches 12MPa, the permeability recovery value of drilling fluid system may be increased to more than 80%. The field application results show that the skin factor is zero in D7H well testing, production is 6.0×105m3/d, exceding proraton produciton nearly 3 times, good reservoir protection effects is received, which provides important reference for subsequent large-scale development of offshore high temperature and high pressure horizontal well.