The electronic ground and excited states of an asymmetric porphycene, 9-amino-2,7,12,17-tetraphenylporphycene (9-ATPPo), are investigated by electronic structure calculations. Different tautomers are considered to address their contributions to the photophysics of 9-ATPPo. Tautomerization pathways on the ground and excited states are constructed between different isomers. It is found that two trans tautomers are mainly responsible for the absorption and emission spectra of 9-ATPPo. These calculations provide a molecular mechanism to explain recent experimental observations, which show a highly complex Q-band structure in the absorption spectrum and pronounced dual fluorescence in the emission spectrum. Furthermore, the current work shows that tautomerization takes place under the assistance of cavity deformations and that a nonradiative process occurs through weak interstate nonadiabatic couplings near the S₁