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 crosslinks. 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.