An anilino substituted rhodamine analogue named 9-(2′-carboxyphenyl)-6-(N,N-diethylamino)-4-(n-phenyl-methanamine)-1,2,3-trihydroxanthylium (1) is investigated by DFT/TDDFT approaches to map its Large Stokes Shift (LSS) and Fluorescence Quantum Yields (FQY) mechanisms. According to PES analysis, both PICT and TICT occur in photoexcitation of 1. The calculated UV–Vis and fluorescence spectra reveal that both PICT and TICT cause LSS. The calculated dual fluorescence indicates 1 can be adequately described by the two state (LE + ICT) mechanism. The calculated Transient Absorption Spectra (TAS) demonstrate that triplet state of 1 may decay from either PICT or TICT through Intersystem Crossing (ISC). With analysis of spontaneous emission, TICT has low FQY, while PICT has high FQY. The research indicates that both PICT and TICT are mechanisms for LSS in 1 and occurrence of PICT is important for increasing FQY for 1.