Conventional polyurethane (PU) thermosets are built by nonrenewable petrifaction resources and are burdensome to be recycled, reprocessed, and reshaped in virtue of their permanent covalent cross-links. Here, for the first time, we synthesized a novel acetal precursor from abundant and renewable furfural and produced PU covalent adaptable networks (CANs) combining excellent reprocessability, controlled degradability, as well as outstanding high-temperature creep resistance. The properties could be further adjusted from soft to hard via the Diels–Alder reaction with bismaleimide. In addition, a small-molecule model study and theoretical calculations with density functional theory (DFT) certified that acetal linkage substituted by a furan ring (or a benzene ring) went through a dissociative exchange reaction via a carbocation mechanism. We envisage that the uncovering of the catalyst-free metathesis of acetal linkage substituted by a furan ring enriches the dynamic chemistry of acetal and greatly promotes the development of acetal-based CANs. Meanwhile, the acetal CANs provide a prime example to foster the development of advanced thermosets from renewable bioresources.