This work has looked to explore an innovative and powerful visible fluorescence immunoassay method through wet NH3-triggered structural change of NH2-MIL-125(Ti) impregnated on paper for the detection of carcinoembryonic antigen (CEA). Gold nanoparticles heavily functionalized with glutamate dehydrogenase (GDH) and secondary antibody were used for generation of wet NH3 with a sandwiched immunoassay format. Paper-based analytical device (PAD) coated with NH2-MIL-125(Ti) exhibited good visible fluorescence intensity through wet NH3-triggeried structural change with high accuracy and reproducibility. Moreover, NH2-MIL-125(Ti)-based PAD displayed two visual modes of fluorescence color and physical color with the naked eye and allowed the detection of CEA at a concentration as low as 0.041 ng mL-1. Importantly, the PAD-based assay provides promise for use in the mass production of miniaturized devices and opens new opportunities for protein diagnostics and biosecurity.