The invention discloses a peat/polyethyleneglycol-grafted composite phase-change energy storage material and a preparation method thereof. The preparation method comprises the following steps: adding aromatic or aliphatic diisocyanate into polyethyleneglycol, reacting for some time, and adding hydroxyacrylate and the like with equal mole to diisocyanate and the like to obtain a hydroxyacrylate-terminated polyethyleneglycol prepolymer; and intensely stirring the hydroxyacrylate-terminated polyethyleneglycol prepolymer to be dispersed in a peat-water system, and dropwisely adding a peroxide initiator and vinyl siloxane to carry out graft polymerization reaction, thereby obtaining the peat/polyethyleneglycol-grafted composite phase-change energy storage material. The composite phase-change energy storage material disclosed by the invention has the advantages of stable phase-change energy storage performance, favorable high/low-temperature properties, water resistance, high enthalpy and aging resistance, and can be widely used in the fields of building thermal insulation and the like.