The presence of additives has demonstrated strong effects on the crystallization and morphology of calcium carbonate (CaCO3). To understand the mediating function of alginate on the growth of CaCO3, we design a novel method to add alginate molecules and inorganic ions mildly and continuously to the mineralization system: a Ca-alginate gel is used as a slow-releasing source of calcium ions and alginate molecules, the gel is gradually broken down by the diffusion of CO2 to the solution, inducing the slow release of Ca2+ and alginate molecules. The slowly released alginate is involved in the nucleation and growth of CaCO3, in the form of micro-sized lens-like particles with a vaterite polymorph and composed of fused nanoparticles. With the increasing reaction time, the lens-like CaCO3 particles gradually develop into a hollow structure and finally turn into ring-shaped CaCO3, in which the polymorph of CaCO3 remains vaterite. The formation of the lens-like particles is the result of the partially-oriented aggregation of primary nanoparticles mediated by alginate. The further evolution of the morphology to ring-shaped particles is due to a dissolution–recrystallization process as well as Ostward ripening.