The invention relates to a preparation method for synthesizing relatively strict monodisperse stable LDHs (layered double hydroxides) colloid nanocrystalline. The preparation method mainly comprises the following steps of: 1, synthesizing the LDHs through a colloid mill to obtain white slurry, then centrifugally washing three times, transferring to a hydrothermal kettle, re-crystallizing to obtain a stable colloid solution; 2, respectively preparing density gradient medium solutions with different densities; 3, sequentially adding quantitative gradient solutions according to different densities in a centrifugal tube for preparing a step type density gradient solution; 4, slowly adding the prepared colloid solution on the liquid level of the density gradient solution, centrifuging under a certain condition; and 5, since colloid nano particles with different sizes have different sedimentation rates in the density gradient solutions, after centrifugation is stopped, the colloid nano particles remain in the gradient solutions with different densities, so that an effective method for preparing the monodisperse LDH nanocrystalline is provided. The high-monodispersity nano material has great potential application on the aspects of medicines, genetic vectors, polymer/LDHs biological nano composite materials, LDH films and the like.