Abstract: In viewing of the formations, lithologic characteristics and the requirements of horizontal wells to drilling fluids in Baobei block in Baolang oilfield, the polysulfon-ate MMH (mixed metal hydroxide) oil emulsion drilling fluid is selected to drill horizontal wells based on study of mud systems used in horizontal well drilling in the world and taking the drilling experiences of vertical wells and deviating wells in this oilfield as reference Field application shows that this drilling fluid can effectively prohibit constriction from formation expansion and has good anti -caving effects, strong suspension carrying capacity and anti - sticking property.