Mn4+-activated red-emitting fluoride phosphors are essen- tial for white light-emitting diodes (WLEDs) with desirable color rendition index (CRI) because of their unique and efficient luminescence characteristics. Herein, we synthesized a novel Mn4+ tactivated dodecfluoride phosphor K3RbGe2F12:Mn4+ (KRGF:Mn) through a facile ionic exchange method at room temperature. A surface-modified strategy using weak reducing agents such as oxalic acid and citric acid is proposed to improve the moisture-resistance ability of KRGF:Mn phosphor dramatically, and the possible mechanism of surface modification has been investigated. A shell formed on the surface of the KRGF:Mn phosphor reduces the concentration of Mn4+ on the surface, which can prevent the internal KRGF:Mn group hydrolysis by the external moisture and effectively decreased the probability of energy migration to surface defects, thereby increasing both the emission efficiency and the moisture-resistance ability of KRGF:Mn. More interestingly, the KRGF:Mn phosphor is quenched after soaking in water for 72 h but recovered to the initial lime (6) brightness after soaking in the modifier solutions for 2 min. This work fabricates a new efficient red phosphor KRGF:Mn for application in warm WLEDs and provides insight into the mechanism of the strategy to improve the moisture resistance of the stability of Mn4+ through surface modification.