To satisfy the requirements of protecting hydrocarbon reservoir of low pressure reservoir horizontal well, by means of research in laboratory, oil-in-water drilling fluid formula with variable density in 0.89~0.99g/cm3was obtained. This oil-in-water lowdensity drilling fluid has good capability of emulsion stability, rheological property and anti-sloughing property. The result of application in Daping #1 well showed that the drilling fluid has stable property and can be simply maintained and adjusted for safe and fast drilling operation. It also can satisfy the demands of near-balanced drilling & underbalanced drilling for low pressure horizontal well and geologic requirement. There are obvious effects for hydrocarbon reservoir protection.