Superconducting (SC) pairing mechanism, origin of high T_c and symmetry of SC order parameter in Fe-based superconductors are among the important unsolved problems in condensed matter and materials physics. We study the SC properties of ThFeAsN, a Fe-based high T_c superconductor, by it ab initio superconducting density functional theory calculations with electron-phonon coupling, screened static and dynamic electron-electron Coulomb repulsion and spin fluctuation (SF) mediated pair-interaction fully taken into account. Our calculations reveal that ThFeAsN is a SF-mediated multiband superconductor with the calculated T_c of 22.4 K and the d_xy-wave SC order parameter with different signs on different Fermi surface sheets, in consistent with experiments. We also present distinct SC properties such as quasiparticle density of states and ultrasonic attenuation coefficient which can be immediately verified by experiments.