There has been much research worldwide on viscosifiers for water-based drilling fluid but research on viscosifiers for oil-based drilling fluid is still in the exploratory stage so that an effective viscosifier for oil-based drilling fluid is lacking. An environmentally friendly viscosifier, KLD-1, for oil-based drilling fluid was developed by reverse emulsion polymerization based on the reaction of non-ionic, anionic, and water-soluble monomers in oil solution. KLD-1 viscosifier was effective at increasing the viscosity and shear stress, resisting temperature, and reducing the biological toxicity and preparation cost (by similar to 12%). It was used successfully in the Fuxin 2 coal field gas well.