The processes of excitation and complicated de-excitation of A-symmetric state in the first-excited band of SO2 molecule were studied experimentally with the techniques of two-photon laser induced dispersive fluorescence spectroscopy where a pulsed dye laser (579 nm) was used as excitation sources. The SO2 molecule which were excited from ground state X1A1 to the high vibrational levels of A1A2 state by absorbing two photons, will realize the repopulation in several vibration-rotational energy levels of A1A2, B1B1 and a3B1 states by internal energy conversion and collision relaxation. Because of transitions to the different vibrational levels of ground electronic state X1A1 from the ground vibrational levels of A1A2, B1B1 and a3B1, the fluorescence spectrum envelopes centered at 305 and 425 nm and the regular fluorescence lines centered at 347.2 nm were formed in the fluorescence spectra. In addition, the process of tri-photon excitation X1A1→C1B2 of SO2 molecule was observed, and the result of the process was the fluorescence spectrum envelope in 200~278 nm and the overlapping fluorescence lines centered at 425 nm. The harmonic frequencies of the symmetry stretch vibration and the bendvibration and the anharmonic constants of stretch vibration mode and the bend vibration mode of related states were calculated from the experimental data.