Crystallization of ions in aqueous micro-droplet or nano-droplet on solid surfaces is ubiquitous, with applications ranging from inkjet printing to pesticide spraying. The substrates involved are typically nonpolar. Yet, the atomistic mechanism of crystallization within sessile droplets on such nonpolar substrates is still unclear. Here, we employ molecular dynamics simulations to investigate the crystallization of sodium chloride inside an aqueous nano-droplet on a nonpolar face-centered-cubic (111) surface. Crystallization occurs inside the droplet rather than at the liquid-gas or solid-liquid interface, when the concentration of the sodium chloride in the droplet exceeds 3.76 mol/kg. The phenomenon originates from the spatial distributions of water molecules and ions: a dense interfacial water layer forms at the solid-liquid interface, whereas ions accumulate in the droplet interior, increasing the local concentration. The ion-water hydration caused by the electrostatic interaction outweighs the ion-solid interaction. The spatial confinement provided by the solid, rather than the physical properties of the solid, enriches ions inside the nano-droplet, thereby triggering the crystallization. We further apply this mechanism to the separated aqueous sodium chloride nanodroplets, in which the gas phase destroys the continuous spatial distribution of ions in the droplet. Analogous crystallization is observed in the sessile droplets of potassium chloride solution on nonpolar solid surfaces, indicating the generality of crystallization in nano-droplets. These findings provide atomic-scale guidance for controlling crystallization in nano-droplets related to microelectronics, inkjet printing, and related technologies.