The effects of mesoporous size and structure on the solidification characteristics of solar salt are simulated by molecular dynamics (MD). The mixed nitrate model with different scales and two structures is established by using Material Studio software, and the model is applied to the Lammps software package for simulation calculation. The changes of freezing point, supercooling, and phase transformation latent heat are summarized. The micro mechanism of solidification characteristics of nano solar salt is analyzed by radial distribution function, potential energy temperature curve and Gibbs free energy theory. The results show that the freezing point of solar salt first increases and then decreases with the increase of nanopore scale. The nanowire structure will also increase the phase transition temperature on the same scale, and the phase transition points of the two eventually tend to be stable with the increase of scale. The supercooling of solar salt decreases with the increase of mesoporous scale, but there is an abnormal increase. Under the two different structures, the solidification enthalpy gradually decreases with the increase of scale, and the phase transition latent heat of nanowire solar salt is 30%–37% higher than that of nanoparticle structure on the same scale.