Regenerated silk fibroin(SF)/Poly(vinyl alcohol)[PVA] blend films were prepared using different casting solvents, water and formic acid, to elucidate the effect of casting solvent on the structure and miscibility of SF/PVA blends. FTIR and XRD measurement suggested that ${eta}$-sheet conformation of SF was not changed by addition of PVA in case of formic acid casting and the casting solvent determined the crystallized component of SF/PVA, leading to a different trend in the overall crystallinity between the two blends. The casting solvent had a dominant role in deciding phase behavior and molecular miscibility of blend films. SEM observation and DMTA measurement elucidated that water solvent produced phase-separated blend films while formic acid yielded one phase blend films with partial miscibility in molecular level indicating that the miscibility of SF blend can be improved by choosing a proper co-solvent.