A comprehensive theoretical study of the reaction mechanisms for the conversion between spiropyrans (SPs) and the open form of merocyanines (MCs) has been conducted by theoretical calculations. The reaction mechanisms on the ground- and triplet-state potential energy surfaces (PESs) were investigated using the density functional method. Time-dependent density functional theory (TD-DFT) calculations using the CIS optimized excited-state geometries were carried out to study the reaction mechanisms on the lowest excited sin-let-state PES. Two possible reaction mechanisms for the thermal conversion between SPs to MCs were found on the ground-state PES. The geometrical parameter, BLA (Bond Length Alternation), which correlates the strengths of the substituents and the polarities of solvents, was used to explain the changes in the reaction mechanism induced by the different donor-acceptor pairs and solvents. In addition, the reaction mechanisms of spiropyranreversible arrowmerocyanine conversion on the triplet and the lowest excited singlet potential energy surfaces were also studied; several possible reaction mechanisms on the excited-state PESs were proposed. A comprehensive mechanistic view of the ultrafast photochemistry of spiropyrans was revealed and interpreted in terms of the strengths of substituents and the polarity of solvents.