Alkali-metal (Li, Na, K)-doped lanthanum oxysulfide nanoplates were obtained in the hexagonal phase by thermolysis of metal acetylacetonates in organic solvents with high boiling points and sulfurization using sulfur powders. The characterization results showed that the size, shape, crystallinity, composition, and phase stability of the La2O2S nanocrystals were affected by the doped alkali metals. Combined with first-principles calculations of the formation energy of anionic defects in the lattices, the ternary phase diagrams for the formation of rare-earth oxysulfides (RE2O2S) were calculated. Both the experimental results and the phase diagrams have demonstrated that the doped alkali metals are capable of promoting the formation of RE2O2S nanocrystals in the order of K≈Na>Li. With K acting as the doping alkali metal, the phase-formation capability for different RE2O2S (e.g., Eu, Gd, Yb, Lu) along the lanthanide series was also explored under the guidance of the hard and soft acid and base (HSAB) theory. Moreover, the as-developed synthetic route could be applied also to the production of K/Eu- and K/Tb-codoped La2O2S nanoplates, which displayed intense red and green emissions, respectively, under ultraviolet-light excitations.