The invention provides a water-based cleaning agent with strong dispersing capacity, and a cleaning method thereof, wherein the cleaning agent comprises narrow-distribution fatty alcohol-polyoxyethylene ether and a nonionic surfactant. According to the invention, the cleaning agent prepared by the preparation method has strong cleaning capability on granular pollutants wrapped by rosin or other lipid compounds, can effectively remove pollutants in difficult-to-clean positions such as deep holes, blind holes and the like, and is simple and stable in components, free of layering, not prone to volatilization and decomposition, free of corrosion effect on welding spots, is wide, inexpensive and easily-available in raw materials, low in cost, low in waste liquid treatment cost, free of toxic substances such as sulfur, chlorine, benzene, heavy metals and the like, and low in harm to human bodies and the environment; the cleaning process is easy to operate, free of residues, high in cleanliness and bright in welding spot; and the application field is wide, and the application prospect is good.