The development of bio-based chain extenders is conducive to the green and sustainable development of the polyurethane industry. In this study, bio-based aromatic ring chain extender 2-hydroxymethyl-5-furaldehyde oxime (HMFO), castor oil polyol 1750 and polymeric 4,4""-diphenylmethane diisocyanate (PMDI) were used as raw materials. By comparing with petrochemical-based aromatic ring chain extender hydroquinone diethylether (HQEE) and non-chain extenders, the influence of bio-based chain extenders on the mechanical properties of thermosetting resins was systematically investigated. The thermosetting polyurethane (TS-PU-HMFO) synthesized with bio-based chain extender HMFO has the best comprehensive performance, with a glass transition temperature of 110.28 ℃, tensile strength, flexural strength, flexural modulus and compressive yield strength of 54.17 MPa, 71.41 MPa, 2155.03 MPa and 50.44 MPa respectively, which are superior to those of thermosetting polyurethane resins with petrochemical-based chain extender HQEE (TS-PU-HQEE) and without chain extender (TS-PU-CK). Among them, the tensile strength is increased by 81.84% and 120.38% respectively, and the flexural strength is increased by 72.36% and 35.94% respectively. This indicates that HMFO as a bio-based aromatic ring chain extender can replace petrochemical-based aromatic ring chain extenders, providing a green and sustainable chain extender for the future development of bio-based thermosetting polyurethanes.