Abstract: Aiming at the under-saturated reservoir and under balanced drilling, taking sea water as external phase and mineral oil (5# white oil) as internal phase, oil in sea water drilling fluid was developed using temperature resistant surface active agent with strong emulsifying power as emulsifier. Performance of the system was evaluated by lab study, and it showed that the drilling fluid have high emulsifying power and reservoir protection capability, which fitted for buried hill reservoirs of Bohai Bay.