The photoionization and dissociative photoionization mechanism of 1,8-dihydroxyanthraquinone (1,8-DHAQ) have been investigated by infrared laser desorption/tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (IR LD/VUV PIMS) technique and theoretical calculations. Consecutive losses of two carbon monoxides and elimination of hydroxyl group are found to be the major fragmentation channels in low photon energy range. Photoionization efficiency (PIE) spectrum of 1,8-DHAQ was measured in the photon energy range of 8.2-15.0 eV. Adiabatic ionization energy (IE) of 1,8-DAHQ (M) and appearance energies (AEs) of the major fragments (M-CO)(+), (M-C2O2)(+), and (M-OH)(+) are determined to be 8.54 +/- 0.05, 10.8 +/-0.1, 11.0 +/- 0.1, and 13.1 +/- 0.1 eV, respectively, which are in fair agreement with calculated results. The B3LYP method with the 6-31+G(d) basis set was used to study fragmentation of 1,8-DHAQ. Theoretical calculations indicate that five lowest-energy isomers of 1,8-DHAQ cations can coexist by virtue of bond rotation and intramolecular proton transfer. A number of decarbonylation and dehydroxylation processes of 1,8-DHAQ cations are well established.