Arginine is considered as a biomarker of cystinuria and other diseases, and thus, it is of urgency to develop a simple and rapid method with high sensitivity and selectivity for arginine detection to meet the demand of on-site analysis and bedside diagnosis. In this work, a lanthanide metal-organic framework, La(TATB), was prepared using a triazine-based planar ligand, 4,4',4″-s-triazine-2,4,6-triyltribenzoate (H3TATB), and lanthanide ion (La3+). La(TATB) can be used as a highly photosensitive agent to activate molecular oxygen to 1O2 to achieve efficient photosensitive oxidation of arginine accompanied by strong blue fluorescence emission under 302 nm UV irradiation. Due to the porous structure and high specific surface area of La(TATB), short-life 1O2 can effectively approach and react with amino acid substrate molecules, thus leading to higher sensitivity than other systems. Therefore, the "turn-on" fluorescence sensing of trace arginine can be realized, with a measured linear response range of 10-20,000 nM and a limit of detection as low as 7 nM. This method can be used for the detection of trace arginine in urine, which is conducive to the bedside diagnosis and rapid screening of cystinuria and other diseases. The proposed method not only expands the application scope of Ln-MOFs but also provides a new construction strategy for "turn-on" luminescence sensors.