A deep understanding of fluorescence turn-off or turn-on mechanisms is fundamental in designing highly effective sensor molecules. However, these mechanisms are subtle as multiple factors may affect the fluorescence signals. Herein, for the first time, we systematically investigated the turn-off fluorescence mechanism upon adding Al3+ cation to a norbornene-derived homopolymer (PNDMIP-DASA). The corresponding monomer (NDMIP-DASA) showed fluorescence enhancement upon the addition of Al3+ ions to THF/DIW solution, while fluorescence turn-off is observed for PNDMIP-DASA. Both NDMIP-DASA and PNDMIP-DASA showed Al3+ detection ability with a low detection limit of 27.60 nM and 2.94 nM, respectively, even in the presence of metal ions. Time-dependent density functional theory (TD-DFT) calculations showed that the PNDMIP-DASA sensor exhibits a large displacement of the potential energy curve between the ground state and the excited state, resulting in an emission shift from the UV-vis to NIR regions. In addition, the oscillator strength of the S1 to S0 transition is close to zero. These results explained the fluorescence sensing mechanism of the turn-off phenomenon. The excellent agreement between calculations and experimental observations also suggests that computational chemistry is a powerful tool to aid the molecular design and engineering of fluorescent compounds.