The excited state intramolecular proton transfer (ESIPT) mechanisms of 1,8-dihydroxydibenzo[a,h]phenazine (DHBP) in toluene solvent have been investigated based on time-dependent density functional theory (TD-DFT). The results suggest that both a single and double proton transfer mechanisms are relevant, in constrast to the prediction of a single one proposed previously (Piechowska et al. J. Phys. Chem. A 2014, 118, 144-151). The calculated results show that the intramolecular hydrogen bonds were formed in the S-0 state, and upon excitation, the intramolecular hydrogen bonds between -OH group and pyridine-type nitrogen atom would be strengthened in the S-1 state, which can facilitate the proton transfer process effectively. The calculated vertical excitation energies in the S-0 and S-1 states reproduce the experimental UV-vis absorption and fluorescence spectra well. The constructed potential energy surfaces of the S-0 and S-1 states have been used to explain the proton transfer process. Four minima have been found on the S-1 state surface, with potential barriers between these excited-state minima of less than 10 kcal/mol, which supports concomitant single and double proton transfer mechanisms. In addition, the fluorescence quenching can be explained reasonably based on the proton transfer process.