An artificial spinning system using regenerated silk fibroin solutions is adopted to produce high-performance silk fibers. In previous studies, alcohol-based agents, such as methanol or ethanol, were used to coagulate silk dope solutions, producing silk fiber with poor mechanical properties compared with those of native silk fibers. The alcohol-based coagulation agents induce rapid beta-sheet crystallization of the silk molecules, which inhibits subsequent alignment of the beta-sheet crystals. Here, we induce gradual beta-sheet formation to afford adequate beta-sheet alignment similar to that of native silk fiber. To this aim, we developed an amorphous silk fiber spinning process that prevents fast beta-sheet formation in silk molecules by using tetrahydrofuran (THF) as a coagulation solvent. In addition, we apply postdrawing to the predominantly amorphous silk fibers to induce beta-sheet formation and orientation. The resultant silk fibers showed a 2.5-fold higher extensibility, resulting in 1.5-fold tougher silk fibers compared with native Bombyx mori silk fiber. The amorphous silk fiber spinning process developed here will pave the way to the production of silk fibers with desired mechanical properties.