3-hydroxyl flavone (3-HF), a typical representative of the second generation new fluorescence molecular probe, was studied by high-accuracy quantum chemical methods, density functional theory (DFT), in the present paper. The photo-physics mechanism of 3-HF as fluorescence molecular probe was studied by the calculation of the optimum geometry structure, charge population and excitation spectrum, in the ideal condition. Then the calculation results were analyzed, and the photo-physics looping graph was plotted for the elucidation of the mechanism. Compared with the experiments, the geometry structure obtained by our calculation is in good agreement, and the errors in the calculated excitation spectrum are in a reasonable limitation. As in this paper, it is only a tentative research work in the field of molecule fluorescence probe with the method of quantum chemistry calculation, and we hope that it could provide systematically valuable theoretical reference in this field in the future.