The structural and dynamical properties of phase I of solid ammonia borane were investigated by means of extensive density functional theory calculations. Ab initio molecular dynamics simulation indicates that molecules in phase I are dynamically tilting along the c axis rather than aligning parallel to the c axis. A 12-site static model of phase I was proposed which is different from former models. Our simulation also demonstrates that the hydrogen distribution in phase I is 4-fold instead of the previously suggested 8-fold or 12-fold. The link between dihydrogen bonding network and lattice structure is discussed.