The dynamic process of 7-hydroxyquinoline with excited state proton transfer has been studied by the pump-probe method in which a three-dimensional degenerate four-wave mixing technique is employed for the probe. The molecular hyperpolarizability and lifetime of the excited state involved in the excited state proton transfer process of 7-hydroxyquinoline are obtained in our experiment. The decay time of the cation excited state is 35 ps and the decay time of the anion excited state is 250 ps. The molecular hyperpolarizability of 7-hydroxyquinoline in the enol form ground state, cation excited state and anion excited state are 1.3 +/- 0.2 X 10(-31), 8.0 +/- 1.0 X 10(-31) and 5.8 +/- 0.8 X 10(-30) esu, respectively.