Developing a portable device for visual and on-site detection of fluoride in groundwater is highly anticipated. In this paper, 2-(tert-butyl-diphenylsilanyloxy)-5-nitro-1H-benzoimidazole (1) has been rationally designed via a silanization reaction for self-calibration detection of fluoride, and the detection limit was calculated as 0.11 ?M. The contact of 1 with fluoride would induce the cleavage of Si-O bond and trigger the emergence of excited state intramolecular proton transfer (ESIPT) process, and then the enol-like emission at 437 nm decreased accompanying with the increase of keto-like tautomerism emission at 550 nm. More importantly, considering the demand of field detection for fluoride in groundwater and combining the function of smartphone to obtain the chroma of photos. The chroma value of the fluorescence color changes from blue to yellow could be conveniently determined through a color recognizer application installed in smartphone. The device can accurately reflect the concentration of fluoride by analyzing the chroma value. The test in actual water samples confirmed that the simple device based on smartphone could be used efficiently for visual, on-site and accurate detection of fluoride in groundwater.