With the trend toward lighter and thinner flexible electronics, developing foldable polymeric substrates that can withstand ultralow folding radiuses has become an urgent issue. Here, a strategy to develop polyimide (PI) films with excellent dynamic and static folding resistance under an ultralarge curvature through copolymerizing one "unidirectional diamine" with classic PMDA-ODA PIs to achieve a kind of folding-chain PI (FPI). It was theoretically and experimentally confirmed that the spring-like folding structure equipped PI films with an enhanced elastic behavior and thus an excellent ability to endure a large curvature. Among them, FPI-20 did not show any crease even after folding over 200000 times under a folding radius of 0.5 mm, while creases were observed on pure PI film only after folding 1000 times. It is noteworthy that the folding radius was almost 5 times smaller than that in current reports (2-3 mm). Meanwhile, the spread angle of FPI-20 films after static folding at 80 °C under a 0.5 mm folding radius get 51% larger than that of films, showing the remarkable static folding resistance.