Lattice composites show excellent mechanical and acoustic properties. Compared with traditional man-made lattice composites, natural (or living) lattice composites exhibit the ability to spontaneously increase their stiffness as time increases, i.e., self-enhancement. With this paper, we study the mechanism of the self-enhancement behavior of living lattice composites. We first immerse a polymeric lattice in an oversaturated CaCO3 solution to simulate the self-enhancement behavior of living lattice composites. We then propose a modeling framework to quantitatively describe the evolution of the morphology and effective stiffness of the growing composites, including a phase field model simulation, a crystal growth prediction, and a modified lattice mechanics theory. We validate the modeling work through comparison among the theoretical prediction, experimental observation, and finite element simulation. We also study the effects of the cross sections of polymeric beams, initial concentration of the solution, and architecture type on the self-enhancement behavior of the composites. This paradigm is expected to open promising avenues for the design and fabrication of synthetic living lattice composites.