The effect of alkali ions (Li⁺, Na⁺, and K⁺) on controllable synthesis of rare earth (RE) fluorides was investigated by introducing corresponding nitrates into hydrothermal systems. It was found that the products transformed from YF₃ truncated octahedra to α-NaYF₄ spheres and then to β-NaYF₄ microrods as the amount of NaNO₃ increased. With the increasing amount, the additive Na⁺ had the effect on controlling the size of α-NaYF₄ particles to decrease. Different alkali ions have distinct abilities of forming corresponding alkali RE fluorides based on their different ionic radii. Possible mechanisms of the alkali cation effect were proposed. By doping Yb³⁺/Tm³⁺ ions, upconversion luminescent properties were also investigated.