The first-principles plane-wave pseudopotential method based on the density functional theory is used to investigate the mechanism of the influence of interaction between interstitial H atom defect and doped atom on the dehydrogenation performance of LiNH2. We obtain the most stable structure of LiNH2 by geometrical optimization, and calculate the binding-energies, interstitial H atom defect formation energies, densities of states (DOSs), and electric charge populations for LiNH2 and doped LiNH2. Studies show that the results of binding-energy cannot reflect the dehydrogenating properties of LiNH2 and doped LiNH2. In equilibrium, there are a number of interstitial H atom defects; the formation energy of interstitial H atom defect is reduced by doping Mg and Ti, which increases the concentration of interstitial H atoms. Interstitial H atoms can induce the defect energy level in the gap, which reduces the width of the gap, and improves the dehydrogenation performance of LiNH2. The strength of N-H bond in NH2- is weakened by interstitial H atom, so that hydrogen atoms in LiNH2 is relatively easy to release. The covalent bond between interstitial H atom and N atom of NH2- explains the escape of NH3 from the dehydrogenation reaction of LiNH2 system. The strengths of N-H bonds are not equal in doped LiNH2, a part of N-H bonds are weaker, and other N-H bonds are strong, the hydrogen atoms are easy to release from weaker N-H bonds.