First-principles calculations were carried out to investigate the crystal structures, band structure, density of states, partial densities of states, Mulliken population and elastic properties of two BeP_{2}N_{4} polymorphs namely phenakite and spinel. The generalized gradient approximation (GGA) and local density approximation (LDA) were used. The calculated results agree well with the experimental data and other theoretical calculations. The electronic structures of BeP_{2}N_{4} indicate that they are insulators with wide bandgaps. The mechanical moduli of the spinel structure are larger than that of phenakite. The hardness of the two polymorphs was evaluated based on the methods proposed by Sung and Gao *et al*. Although the bulk modulus of phenakite is small the results indicate that it is a relatively hard material. On the other hand, the spinel structure is a super hard phase. When the pressure increases the phenakite structure gradually becomes malleable. The calculated GGA transition pressure from phenakite to spinel is 14 GPa, which is smaller than the predicted value of 24 GPa.