High purity green up-conversion (UC) materials have wide applications in fluorescent labeling, plastic recycling and displays, etc. However, Yb³⁺/Er³⁺-based UC materials exhibit poor color purity of green light, and their output color is normally unstable under a variable excitation power density, which hinders their application in the aforementioned areas. Herein, we report the highly stable and efficient pure green UC of rod-like β-NaGdF₄:Yb³⁺,Ho³⁺ submicro-crystals synthesized via a newly developed ion-exchange modified (IEM) method. IEM-derived NaGdF₄:Yb³⁺,Ho³⁺ monodisperse submicro-crystals present not only a high-purity green output color (due to the large intensity ratio of green to red, I_Green/I_Red ≈ 5) but also excellent color stability under varying conditions, such as excitation power density and doping concentration of Yb³⁺/Ho³⁺. More importantly, IEM arouses an enormously enhanced UC emission intensity of ∼3700-fold that of the precursor crystals (PC). The UC emission intensity of IEM β-NaGdF₄:Yb³⁺,Ho³⁺ is ∼4 times stronger than that of its high temperature annealing (HTA) counterpart. The emission intensity of the single green band is ∼40% stronger than that of the IEM β-NaGdF₄:20%Yb³⁺,2%Er³⁺ submicro-crystals. A proof-of-concept fluorescent labeling of clover patterns (15 × 12 mm) printed from NaGdF₄:Yb³⁺,Ho³⁺ ink with stable and high purity green fluorescence is demonstrated under excitation with various power densities.