Recently, oil-based drilling fluids are used mainly in horizontal wells, which are highly cost and have a poor performance to carry cuttings and may result in environmental problems because of the ineffective dispose of drilling waste. Therefore, a study is commenced to develop a water-based drilling fluid system (MEG drilling fluid system) that can satisfy the needs of horizontal well. The study begins with the molecular structure and properties of monomers about MEG. Then it selected the treatment agent which has a good compatibility with MEG, and confirms a best formula of MEG drilling fluid system. By the comparison between MEG and other drilling fluids, the former has evident advantages in cave preventing, lubricity, solid carrying and formation damage controlling; also it can minimize the environmental effects. The above proves that MEG drilling fluid system can well meet the need for horizontal well drilling.