The invention relates to a polyolefin composition, in particular to a rotomolded polyolefin composition and a preparation method thereof. The rotomolded polyolefin composition comprises polyethylene,hyperbranched polyethylene, a nucleating agent, an acid absorbent, an ultraviolet absorbent, and an antioxidant, wherein the hyperbranched polyethylene, the nucleating agent, the antioxidant and the acid absorbent are pretreated; to be specific, the hyperbranched polyethylene, the nucleating agent, the antioxidant and the acid absorbent are mixed, n-hexane is added, a solvent is removed after heating and reflowing, and drying is performed. The rotomolded polyolefin composition and the preparation method thereof have the advantages that mechanical properties of rotomolding materials can be significantly improved, melt viscosity (low zero-cut viscosity) is reduced, the produced rotomolded product has smooth inner and outer surfaces and good plasticity, the section of the product has no evident bubbles, heating time is short, demolding is facilitated, and the product has long-term ultraviolet ageing resistance. The invention also provides the preparation method of the rotomolded polyolefin composition.