Abstract: The reservoir forming condition of buried-hill is complex in Nanpu Oilfield, and the vertical and horizontal heterogeneity is very strong. In order to effectively raise fracture penetration rate and achieve economic development, underbalanced drilling technology for horizontal wells was researched and applied. For the following drilling risk situations, such as: deeply buried, high temperature, high gas-oil ratio, narrow density window, prone to lost circulation and well kick, etc, a series of studies are conducted，such as optimizing a temperature resistant oil-in-water drilling fluid system, analyzing the gas-oil ratio and the application of real-time pressure monitoring and reservoir evaluation technology, designing the underbalanced and managed pressure drilling parameters, monitoring dynamic and static temperature variation and the abnormal characteristics when drilling buried-hill fracture, a set of underbalanced drilling technology was developed for horizontal wells in deep buried-hill reservoirs with high temperature in Nanpu oilfield. The results of application show that the technology is able to ensure safe drilling and economic development for buried-hill reservoirs in Nanpu Oilfield, and also to give references for drilling the same type of reservoirs.