Uniform lens-like vaterite mesocrystals (LVM) were formed in a hydrosoluble polymer network by combining branched polyethyleneimine (BPEI) with maleic acid (MAc) through hydrogen bonds and electrostatic interaction to stimulate the mild biomineralization occurred in nature. Various characterization methods were employed to investigate the formation mechanism of LVM. The results of scanning electron microscope (SEM) and transmission electron microscope (TEM) depicted the evolution process of LVM: the initial nucleation of amorphous calcium carbonate (ACC) in the supersaturation solution, the subsequent aggregation process, and the final crystallization from ACC nuclei to vaterite nanocrystals and their oriented attachment into uniformly mesocrystals. Furthermore, the decreasing organic constitutions were involved in the transformation from amorphous precursors into final mesocrystals, confirming the expulsion of the organic additives during the transformation process and in accompany with increasing crystallinity degrees with prolonged reaction time. This biomimetic mineralization strategy of LVM in mild condition is similar to that of the nacre formation in nature, and can illuminate biomaterials research such as calcium phosphate and bio-glass production.