In order to predict the crystal density of all-nitrogen materials accurately, the 'specific' molecular force field of all-nitrogen materials was established based on the quantum chemistry calculation method. The crystal densities of twenty kinds of all-nitrogen molecules were calculated by predicting the crystal packing structure. Results show that the crystal densities for five kinds of all-nitrogen molecules with cage type including N₄(T_d), N₆(D_3h), N₈(O_h), N₁₀(D_5h), and N₁₂(D_6h) are 1.81, 2.08, 2.47, 2.46 g·cm^-3 and 2.57 g·cm^-3, respectively. As the number of nitrogen atoms increases, the change in crystal densities of all-nitrogen molecules with cage type is different from the progressive trend in literatures, but at N₈(O_h), there is a mutation, which reflects the specific force field parameters.