The invention discloses a water-based drag-reducing agent for a coiled tube of a shale gas horizontal well and a preparation method thereof. The drag-reducing agent comprises the following raw materials: hydroxyethyl cellulose, n-decyl glucoside, oleic acid based hydroxyethyl imidazoline, isothiazolinone, molybdenum disulfide and water. With the drag-reducing agent provided by the invention, the hydroxyethyl cellulose used as a carrier is used for uniformly suspending and dispersing lubricating and drag-reducing components in a product, smoothly carrying the lubricating and drag-reducing components to all layered sections of a horizontal well in the process of application, and synergistically cleaning oil-based drilling fluid residues and high-density solid-phase substances with the n-decyl glucoside; the oleic acid based hydroxyethyl imidazoline has the effects of corrosion inhibition and corrosion prevention, so the drag-reducing agent can better protect coiled tube tools and downhole sleeve tubes in the process of application; the isothiazolinone has the effect of killing bacteria and prevents a suspension dispersant namely the hydroxyethyl cellulose from generating biological degradation in the processes of storage and use; and the molybdenum disulfide has the effects of high-temperature resistance, heavy-load resistance, lubrication and drag reduction.