Living organisms make use of a variety of inorganic and organic components to form biogenic minerals with hierarchical structures and fascinating properties, triggering the development of biomimetic mineralization. The introduction of organic additive is a versatile strategy, and a wide range of organics have already been adopted to mimic biosystems designing and synthesizing advanced functional minerals. Insoluble cellulose is the most abundant polysaccharide in nature, but the insolubility has limited its extensive applications. In this study, we first find that concentrated calcium chloride aqueous solution is an effective solvent for cellulose, and dissolved cellulose plays a pivotal role in directing the formation of ultrathin hydroxyapatite (HA) nanobelts of ca. 10 nm in thickness. To investigate the assembling process of the belt, samples collected at different reaction times were observed. The results indicate that nanoneedles form first, and then they assemble into the prototype of nanobelts by lateral/longitudinal aggregation and arrangement. Subsequently, the nanobelts gradually become dense, transparent, and smooth via crystallographic fusion of adjacent nanoneedles, indicating the highly elaborated evolution of morphology resulted from a time-dependent process. During the evolution of nanobelts, dissolved cellulose is supposed to participate in the mineralization of HA via the bonding of its hydrophilic groups with phosphate groups and calcium ions and the interaction of cellulose molecules with HA crystal planes. These findings provide unique insight into the application of dissolved cellulose in aqueous solution and an inspiration of a bottom-up strategy for designing delicate mineral assemblies directed by insoluble organics.