As a low-density under balance drilling fluid liquid for deep well, oil-in-water drilling fluid is suitable for the construction in low pressure and low permeability formations. By the investigation on development situation of oil-in-water drilling fluid both in China and abroad, the laboratory experiments were made on liquid xanthan gum oil-in-water drilling fluid. According to the different drilling fluid systems, the test was divided into 5 groups to test and study the effect of different additions to emulsifying property and rheology of each drilling fluid system. The experimental results show that with the increase of the liquid xanthan gum amount, the emulsifying property of each drilling fluid system is gradually improved, and so is the rheology. Through the analysis on the test results, the formula of polysulfonate oil-in-water drilling fluid system is optimized, which has good lubricity with simple construction process; and the formula of low density aerated drilling fluid system is optimized, which is suitable for under balance pressure drilling to effectively protect the reservoir.