Provided are a non-sulfonated melamine resin viscosity reducer for a drilling fluid and a preparation method thereof. The non-sulfonated melamine resin viscosity reducer for the drilling fluid includes the following raw materials in parts by mass: 100 parts to 150 parts of water, 80 parts to 100 parts of melamine, 20 parts to 30 parts of formaldehyde, 50 parts to 80 parts of a tannin extract, 40 parts to 60 parts of lignin, 20 parts to 40 parts of maleic anhydride, 0.1 parts to 0.3 parts of a catalyst, 1 part to 3 parts of triethanolamine (TEA), and 10 parts to 20 parts of alcoholamine; wherein the alcoholamine is one or more selected from the group consisting of monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), isopropanolamine (IPA), and diisopropanolamine (DIPA).

A resin composition contains a maleimide compound (A) having a benzene ring in the molecule and a maleimide equivalent of 500 g/mol or less; an imide compound (B) having at least one of a hydrocarbon group or a maleimide group at the molecular end; and a radical polymerizable compound (C) having a benzene ring to which an alkenyl group is bonded in the molecule and a weight average molecular weight of 1,000 or less.