Abstract: The Mesozoic reservoir of Zhenjing area in southern Ordos basin is low porosity, low permeability and low productivity sandstone reservoir. Oil layers are easy to be injured during drilling and completion, and some wells have no natural productivity if no fracturing. In order to protect reservoir and enhance productivity, through the research of reservoir protection, laboratory testing and field testing, the K + - NH4 + drilling fluid system is concluded which is suitable for low - permeability reservoir of Mesozoic. The system has the characteristics of low damage, good inhibition and rheological property. At present, the system has been popularized in Zhenjing area and achieved great application result.