Although the cost of oil base drilling fluid is extremely high, it always has incomparable advantages compared with water base drilling fluid for the complex formation drilling in offshore. According to the difficulties encountered in 3 complex blocks of an oil and gas field in western South China Sea, corresponding oil base drilling fluid formula, performance and maintenance methods are presented, the processing methods for oily drilling cuttings in different sea areas are discussed. The drilling practice proves that oil base drilling fluid is not only a very important guarantee for smooth drilling operation in easy-collapsed wells, HTHP wells and super-low pore pressure adjustment wells in old oil field, but also has good reservoir protection performance for oilfield exploration and development.