Controlling the molecular assembling and nanomorphology of organic semiconductors is crucial to obtain high-performance electronic devices. In this work, we have first reported novel superlong salicylideneaniline nanobelts (mHBA) using the magnetic nanoparticle-assisted self-assembly process. Our results show that magnetic nanoparticles will obviously influence the self-assembly behavior, nanomorphology, and crystal structure of molecular HBA. Moreover, the intensity of fluorescence mHBA exhibits decreasing and increasing patterns, with the increase in temperature over a wide temperature range of 8 to 295 K. To elucidate the origin of tautomer forms, the ground and excited states of mHBA were experimentally and theoretically studied. Our results suggest that superlong HBA nanobelts provide a promising intelligent fluorescent thermometer and an organic field-effect transistor.