Abstract: The borehole wall sloughing easily happens during drilling due to the unconsolidated formation with micro-fracture development in Tambococha Oilfield, Ecuador. The target stratum is characterized by high porosity and high permeability, causing reservoir damage. For this reason, with polyalcohol XCS-III and emulsified paraffin G325 as the main agents, accompanied by optimized eco-friendly agents, water-base drilling fluid that would provide strong reservoir protection during plugging were studied. The drilling fluid utilized the synergistic plugging effect of emulsified droplets, cloud point effect, micron particles and rigid particles to reduce the oil-water interfacial tension. It prevented the drilling fluid from entering the formation, finally achieving anti-collapse plugging and reservoir protection. Laboratory experiments showed that the drilling fluid had strong inhibitory properties, reducing the linear expansion of cuttings by 64.8% compared with that in clean water. The plugging performance was good, and its particle size distribution was consistent with the ideal filling curve. The damage to the reservoir was minor with an average damage rate of 10.25% to the cores of Layer U in the Napo Formation. The drilling fluid was eco-friendly, and the content of heavy metals met the standards for sewage discharge. The drilling fluid system was successfully applied to 15 horizontal wells in Tambochocha Oilfield, achieving good wellbore stability and smooth tripping. The average daily crude oil output of a single well exceeded 300 t, which was nearly 90% higher than that of offset wells. Such envi-ronmental indicators as waste drilling fluid and drilling cuttings met the requirements of EPA1311 discharge standards. The research results showed that the water-base drilling fluid could meet the rate of penetration (ROP) improvement and reservoir protection requirements of horizontal wells in Tambococha Oilfield, Ecuador.