Three kinds of composites were built by nitrogen-doped graphene and phosphomolybdic acid. Based on the density functional theory (DFT), the combined energies, charge populations, orbital distributions and densities of states (DOS) were calculated. The calculated results show that the short-range interactions can be formed by oxygen atoms and nitrogen atoms, and the charge can be transferred from phosphomolybdic acid to graphene. It is found that the conductive bands (CB) of phosphomolybdic acid move to the lower-energy level. At the same time, more valence bands (VB) were found in DOS of three composites. Finally, the reason for the excellent capacitive ability of the composites is revealed. That is, nitrogen-doped graphene can improve the capacitive ability of the material by receiving the excessive electrons on surface of phosphomolybdic acid.