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Investigation of efficient red phosphors is highly desired for the development of novel warm white light emitting diodes. In this paper, through a facile method, we first introduce K+ to replace Ba2+ to synthesize a novel K2xBa1-xTiF6:Mn4+ solid solution phosphor with high luminous efficiency, high thermal stability and good waterproofness. Significantly, compared with two best luminescence performance samples (BaTiF6:0.006Mn4+ and K0.070Ba0.965TiF6:0.018Mn4+), the luminescence intensity of K0.070Ba0.965TiF6:0.018Mn4+ is increased by 6 times, the quantum efficiency is improved from 40% to 75% and the activation energy of thermal quenching is improved to 0.940?eV from 0.628?eV. Remarkably, the waterproofness is improved by 45% than traditional red phosphor K2TiF6:Mn4+ after being immersed in water for 150?min. Moreover, a high-performance WLED with low correlated color temperature (CCT?=?3930?K), high color rendering index (Ra?=?90.3) and luminous efficacy of 105 lm/W is achieved by employing blue-emitting GaN chip, yellow-emitting YAG:Ce3+ phosphor and red-emitting K0.070Ba0.965TiF6:Mn4+ phosphor. These results reveal that introduced suitable cation may have a positive effect on luminescent properties of Mn4+ activated fluoride phosphors, which is a helpful tool for their performance improvement, and K0.070Ba0.965TiF6:Mn4+ presents a good luminescent property and may be a potential candidate for high-performance warm WLED.