The electronic and spectral properties of p-tertbutylthiacalix[4]arene (TCA) and its Cu2+ complexes (CuTCA and Cu(2)TCA) were characterized by time-dependent density functional theory (TD-DFT). Geometries of TCA, CuTCA, and Cu(2)TCA were optimized at the CAM-B3LYP/6-31+G(d,p) level of theory in water using the conductor-like polarizable continuum model. The absorption spectra of TCA, CuTCA, and Cu(2)TCA were demonstrated by TD-DFT method. The quenching mechanism of perylene as the fluorophore of a chemosensor based on thiacalix[4]arene was discussed. The addition of Cu2+ to TCA introduced a series of low-lying excited states involving copper d orbitals. The overlap between absorption of TCA complexes and emission of perylene indicated that the quenching of perylene fluorescence is due to fluorescence resonance energy transfer.