Room-temperature phosphorescence (RTP) from pure organic materials has been promising in next-generation OLEDs. Understanding the photophysical properties of RTP molecules is attractive but challenging. In this study, through a combined quantum mechanics and molecular mechanics (QM/MM) method taking 2-(3,4-dimethoxybenzyl)isoindoline-1,3-dione (complex b) as an example, we comparatively investigate the photophysical properties of complex b in diverse environments (solution, crystal, and amorphous). From solution to amorphous to crystal phase, the excited-state decay rates for the molecule indicate that the AIE phenomenon of complex b is mainly induced by the increased phosphorescence rates. However, the increased nonradiative decay rate knr of T1 → S0 from the solution to the crystal phase could be attributed to the different electron coupling in the crystal phase. Moreover, the theoretical results also show that the small energy gap between the lowest singlet excited state (S1) and triplet excited state (T1) and low reorganization energy can help enhance intersystem crossing to facilitate a more competitive radiative process from the T1 state to ground state (S0). Additionally, the stronger intermolecular π-π interaction can cause high phosphorescence quantum efficiency in the crystalline phase. Our study presents a rational explanation for aggregation-induced RTP, which is beneficial for the design of new organic RTP materials in the future.